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Professor Mark Gieles


Professor of Astrophysics and Royal Society University Research Fellow (URF)
+44 (0)1483 683171
18 BC 03

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Astrophysics Research Group.

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My publications

Publications

Bastian N, Konstantopoulos IS, Trancho G, Weisz DR, Larsen SS, Fouesneau M, Kaschinski CB, Gieles M (2012) Spectroscopic Constraints on the Form of the Stellar Cluster Mass
Function,
Astronomy and Astrophysics
This contribution addresses the question of whether the initial cluster mass
function (ICMF) has a fundamental limit (or truncation) at high masses. The
shape of the ICMF at high masses can be studied using the most massive young
( statistics. In this contribution we use an alternative method based on the
luminosities of the brightest clusters, combined with their ages. If a
truncation is present, a generic prediction (nearly independent of the cluster
disruption law adopted) is that the median age of bright clusters should be
younger than that of fainter clusters. In the case of an non-truncated ICMF,
the median age should be independent of cluster luminosity. Here, we present
optical spectroscopy of twelve young stellar clusters in the face-on spiral
galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and
the brightness of the clusters is taken from the literature. The observations
are compared with the model expectations of Larsen (2009) for various ICMF
forms and both mass dependent and mass independent cluster disruption. While
there exists some degeneracy between the truncation mass and the amount of mass
independent disruption, the observations favour a truncated ICMF. For low or
modest amounts of mass independent disruption, a truncation mass of 5-6*10^5
Msun is estimated, consistent with previous determinations. Additionally, we
investigate possible truncations in the ICMF in the spiral galaxy M83, the
interacting Antennae galaxies, and the collection of spiral and dwarf galaxies
present in Larsen (2009) based on photometric catalogues taken from the
literature, and find that all catalogues are consistent with having a
(environmentally dependent) truncation in the cluster mass functions.
Pijloo JT, Zwart SFP, Alexander PER, Gieles M, Larsen SS, Groot PJ, Devecchi B (2015) The initial conditions of observed star clusters - I. Method description and validation, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 453 (1) pp. 605-637 OXFORD UNIV PRESS
Gieles M (2009) The early evolution of the star cluster mass function, MON NOT R ASTRON SOC
Several recent studies have shown that the star cluster initial mass function
(CIMF) can be well approximated by a power law, with indications for a
steepening or truncation at high masses. This contribution considers the
evolution of such a mass function due to cluster disruption, with emphasis on
the part of the mass function that is observable in the first ~Gyr. A Schechter
type function is used for the CIMF, with a power law index of -2 at low masses
and an exponential truncation at M*. Cluster disruption due to the tidal field
of the host galaxy and encounters with giant molecular clouds flattens the
low-mass end of the mass function, but there is always a part of the `evolved
Schechter function' that can be approximated by a power law with index -2. The
mass range for which this holds depends on age, t, and shifts to higher masses
roughly as t^0.6. Mean cluster masses derived from luminosity limited samples
increase with age very similarly due to the evolutionary fading of clusters.
Empirical mass functions are, therefore, approximately power laws with index
-2, or slightly steeper, at all ages. The results are illustrated by an
application to the star cluster population of the interacting galaxy M51, which
can be well described by a model with M*=(1.9+/-0.5)x10^5 M_sun and a short
(mass-dependent) disruption time destroying M* clusters in roughly a Gyr.
Sabbi E, Lennon DJ, Gieles M, De Mink SE, Walborn NR, Anderson J, Bellini A, Panagia N, Van Der Marel R, Apellániz JM (2012) A double cluster at the core of 30 Doradus, Astrophysical Journal Letters 754 (2)
Based on an analysis of data obtained with the Wide Field Camera 3 on the Hubble Space Telescope we report the identification of two distinct stellar populations in the core of the giant H II region 30 Doradus in the Large Magellanic Cloud. The most compact and richest component coincides with the center of R136 and is 1 Myr younger than a second more diffuse clump, located 5.4 pc toward the northeast. We note that published spectral types of massive stars in these two clumps lend support to the proposed age difference. The morphology and age difference between the two sub-clusters suggests that an ongoing merger may be occurring within the core of 30 Doradus. This finding is consistent with the predictions of models of hierarchical fragmentation of turbulent giant molecular clouds, according to which star clusters would be the final products of merging smaller sub-structures. © 2012. The American Astronomical Society. All rights reserved..
Alexander PER, Gieles M (2013) Constraining the initial conditions of globular clusters using their
radius distribution,
Monthly Notices of the Royal Astronomical Society
Studies of extra-galactic globular clusters have shown that the peak size of
the globular cluster (GC) radius distribution (RD) depends only weakly on
galactic environment, and can be used as a standard ruler. We model RDs of GC
populations using a simple prescription for a Hubble time of relaxation driven
evolution of cluster mass and radius, and explore the conditions under which
the RD can be used as a standard ruler. We consider a power-law cluster initial
mass function (CIMF) with and without an exponential truncation, and focus in
particular on a flat and a steep CIMF (power-law indices of 0 and -2,
respectively). For the initial half-mass radii at birth we adopt either
Roche-lobe filling conditions ('filling',meaning that the ratio of half-mass to
Jacobi radius is approximately rh/rJ ~ 0.15) or strongly Roche-lobe
under-filling conditions ('under-filling', implying that initially rh/rJ 0.15). Assuming a constant orbital velocity about the galaxy centre we find for
a steep CIMF that the typical half-light radius scales with galactocentric
radius RG as RG^1/3. This weak scaling is consistent with observations, but
this scenario has the (well known) problem that too many low-mass clusters
survive. A flat CIMF with 'filling' initial conditions results in the correct
mass function at old ages, but with too many large (massive) clusters at large
RG. An 'underfilling' GC population with a flat CIMF also results in the
correct mass function, and can also successfully reproduce the shape of the RD,
with a peak size that is (almost) independent of RG. In this case, the peak
size depends (almost) only on the peak mass of the GC mass function. The (near)
universality of the GC RD is therefore because of the (near) universality of
the CIMF. There are some extended GCs in the outer halo of the Milky Way that
cannot be explained by this model.
Bastian N, Gieles M (2006) Cluster Disruption: Combining Theory and Observations,
We review the theory and observations of star cluster disruption. The three
main phases and corresponding typical timescales of cluster disruption are: I)
Infant Mortality (~10^7 yr), II) Stellar Evolution (~10^8 yr) and III) Tidal
relaxation (~10^9 yr). During all three phases there are additional tidal
external perturbations from the host galaxy. In this review we focus on the
physics and observations of Phase I and on population studies of Phases II &
III and external perturbations concentrating on cluster-GMC interactions.
Particular attention is given to the successes and short-comings of the Lamers
cluster disruption law, which has recently been shown to stand on a firm
physical footing.
Renaud F, Gieles M (2013) The role of galaxy mergers on the evolution of star clusters, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 431 (1) pp. L83-L87 OXFORD UNIV PRESS
Gieles M, Zocchi A (2015) A family of lowered isothermal models, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 454 (1) pp. 576-592 OXFORD UNIV PRESS
Gieles M (2012) Dynamical expansion of star clusters, Astrophysics and Space Science Proceedings pp. 241-243
Most globular clusters have half-mass radii of a few pc with no apparent correlation with their masses. This is different from elliptical galaxies, for which the Faber-Jackson relation suggests a strong positive correlation between mass and radius. Objects that are somewhat in between globular clusters and low-mass galaxies, such as ultra-compact dwarf galaxies, have a mass-radius relation consistent with the extension of the relation for bright ellipticals. Here we show that at an age of 10Gyr a break in the mass-radius relation at
Bianchini P, Renaud F, Gieles M, Varri AL (2014) The inefficiency of satellite accretion in forming extended star
clusters,
The distinction between globular clusters and dwarf galaxies has been
progressively blurred by the recent discoveries of several extended star
clusters, with size (20-30 pc) and luminosity (-6 one of faint dwarf spheroidals. In order to explain their sparse structure, it
has been suggested that they formed as star clusters in dwarf galaxy satellites
that later accreted onto the Milky Way. If these clusters form in the centre of
dwarf galaxies, they evolve in a tidally-compressive environment where the
contribution of the tides to the virial balance can become significant, and
lead to a super-virial state and subsequent expansion of the cluster, once
removed. Using N-body simulations, we show that a cluster formed in such an
extreme environment undergoes a sizable expansion, during the drastic variation
of the external tidal field due to the accretion process. However, we show that
the expansion due to the removal of the compressive tides is not enough to
explain the observed extended structure, since the stellar systems resulting
from this process are always more compact than the corresponding clusters that
expand in isolation due to two-body relaxation. We conclude that an accreted
origin of extended globular clusters is unlikely to explain their large spatial
extent, and rather favor the hypothesis that such clusters are already extended
at the stage of their formation.
Bastian N, Adamo A, Schirmer M, Hollyhead K, Beletsky Y, Carraro G, Davies B, Gieles M, Silva-Villa E (2014) The effect of spatial resolution on optical and near-IR studies of
stellar clusters: Implications for the origin of the red excess,
Recent ground based near-IR studies of stellar clusters in nearby galaxies
have suggested that young clusters remain embedded for 7-10Myr in their
progenitor molecular cloud, in conflict with optical based studies which find
that clusters are exposed after 1-3Myr. Here, we investigate the role that
spatial resolution plays in this apparent conflict. We use a recent catalogue
of young ($5000$~\msun) clusters in the nearby spiral
galaxy, M83, along with Hubble Space Telescope (HST) imaging in the optical and
near-IR, and ground based near-IR imaging, to see how the colours (and hence
estimated properties such as age and extinction) are affected by the aperture
size employed, in order to simulate studies of differing resolution. We find
that the near-IR is heavily affected by the resolution, and when aperture sizes
$>40$~pc are used, all young/blue clusters move red-ward in colour space, which
results in their appearance as heavily extincted clusters. However, this is due
to contamination from nearby sources and nebular emission, and is not an
extinction effect. Optical colours are much less affected by resolution. Due to
the larger affect of contamination in the near-IR, we find that, in some cases,
clusters will appear to show near-IR excess when large ($>20$~pc) apertures are
used. Our results explain why few young ($ 1$~mag) clusters have been found in recent ground based near-IR studies of
cluster populations, while many such clusters have been found in higher
resolution HST based studies. Additionally, resolution effects appear to (at
least partially) explain the origin of the near-IR excess that has been found
in a number of extragalactic YMCs.
Gieles M, Bastian N, Lamers HJGLM (2004) The Star Cluster Population of M51,
We present the age and mass distribution of star clusters in M51. The
structural parameters are found by fitting cluster evolution models to the
spectral energy distribution consisting of 8 HST-WFPC2 pass bands. There is
evidence for a burst of cluster formation at the moment of the second encounter
with the companion NGC5195 (50-100 Myr ago) and a hint for an earlier burst
(400-500 Myr ago). The cluster
IMF has a power law slope of -2.1. The disruption time of clusters is
extremely short (
Bastian N, Gieles M, Ercolano B, Gutermuth R (2008) The Spatial Evolution of Stellar Structures in the LMC/SMC, Monthly Notices of the Royal Astronomical Society
We present an analysis of the spatial distribution of various stellar
populations within the Large and Small Magellanic Clouds. We use optically
selected stellar samples with mean ages between ~9 and ~1000 Myr, and existing
stellar cluster catalogues to investigate how stellar structures form and
evolve within the LMC/SMC. We use two statistical techniques to study the
evolution of structure within these galaxies, the $Q$-parameter and the
two-point correlation function (TPCF). In both galaxies we find the stars are
born with a high degree of substructure (i.e. are highly fractal) and that the
stellar distribution approaches that of the 'background' population on
timescales similar to the crossing times of the galaxy (~80/150 Myr for the
SMC/LMC respectively). By comparing our observations to simple models of
structural evolution we find that 'popping star clusters' do not significantly
influence structural evolution in these galaxies. Instead we argue that general
galactic dynamics are the main drivers, and that substructure will be erased in
approximately the crossing time, regardless of spatial scale, from small
clusters to whole galaxies. This can explain why many young Galactic clusters
have high degrees of substructure, while others are smooth and centrally
concentrated. We conclude with a general discussion on cluster 'infant
mortality', in an attempt to clarify the time/spatial scales involved.
Bastian N, Gieles M, Efremov YN, Lamers HJGLM (2005) Hierarchical Star Formation in M51: Star/Cluster Complexes,
We report on a study of young star cluster complexes in the spiral galaxy
M51. Recent studies have confirmed that star clusters do not form in isolation,
but instead tend to form in larger groupings or complexes. We use {\it HST}
broad and narrow band images (from both {\it WFPC2} and {\it ACS}), along with
{\it BIMA}-CO observations to study the properties and investigate the origin
of the e complexes. We find that the complexes are all young ($ sizes between $\sim$85 and $\sim$240 pc, and have masses between 3-30 $\times
10^{4} M_{\odot}$. Unlike that found for isolated young star clusters, we find
a strong correlation between the complex mass and radius, namely $M\propto
R^{2.33 \pm 0.19}$. This is similar to that found for giant molecular clouds
(GMCs). By comparing the mass-radius relation of GMCs in M51 to that of the
complexes we can estimate the star formation efficiency within the complexes,
although this value is heavily dependent on the assumed CO-to-H$_2$ conversion
factor. The complexes studied here have the same surface density distribution
as individual young star clusters and GMCs. If star formation within the
complexes is proportional to the gas density at that point, then the shared
mass-radius relation of GMCs and complexes is a natural consequence of their
shared density profiles. We briefly discuss possibilities for the lack of a
mass-radius relation for young star clusters. We note that many of the
complexes show evidence of merging of star clusters in their centres,
suggesting that larger star clusters can be produced through the build up of
smaller clusters.
Gieles M, Zwart SFP, Baumgardt H, Athanassoula E, Lamers HJGLM, Sipior M, Leenaarts J (2006) Star cluster disruption by giant molecular clouds, Mon.Not.Roy.Astron.Soc. 371 pp. 793-804
We investigate encounters between giant molecular clouds (GMCs) and star
clusters. We propose a single expression for the energy gain of a cluster due
to an encounter with a GMC, valid for all encounter distances and GMC
properties. This relation is verified with N-body simulations of cluster-GMC
encounters and excellent agreement is found. The fractional mass loss from the
cluster is 0.25 times the fractional energy gain. This is because 75% of the
injected energy goes to the velocities of escaping stars, that are higher than
the escape velocity. We derive an expression for the cluster disruption time
(t_dis) based on the mass loss from the simulations, taking into account the
effect of gravitational focusing by the GMC. The disruption time depends on the
cluster mass (M_c) and half-mass radius (r_h) as t_dis=2.0 S (M_c/10^4
M_sun)(3.75 pc/r_h)^3 Gyr, with S=1 for the solar neighbourhood and inversely
proportional to the GMC density. The observed shallow relation between cluster
radius and mass gives t_dis a power-law dependence on the mass with index 0.7,
similar to that found from observations and from simulations of clusters
dissolving in tidal fields (0.62). The constant of 2.0 Gyr is about a factor of
3.5 shorter than found from earlier simulations of clusters dissolving under
the combined effect of galactic tidal field and stellar evolution. It is
somewhat higher than the observationally determined value of 1.3 Gyr. It
suggests, however, that the combined effect of tidal field and encounters with
GMCs can explain the lack of old open clusters in the solar neighbourhood. GMC
encounters can also explain the (very) short disruption time that was observed
for star clusters in the central region of M51, since there rho_n is an order
of magnitude higher than in the solar neighbourhood.
Bastian N, Gieles M, Ercolano B, Gutermuth R (2008) The Spatial Evolution of Stellar Structures in the LMC,
We present an analysis of the spatial distribution of various stellar
populations within the Large Magellanic Cloud. We combine mid-infrared selected
young stellar objects, optically selected samples with mean ages between ~9 and
~1000 Myr, and existing stellar cluster catalogues to investigate how stellar
structures form and evolve within the LMC. For the analysis we use Fractured
Minimum Spanning Trees, the statistical Q parameter, and the two-point
correlation function. Restricting our analysis to young massive (OB) stars we
confirm our results obtained for M33, namely that the luminosity function of
the groups is well described by a power-law with index -2, and that there is no
characteristic length-scale of star-forming regions. We find that stars in the
LMC are born with a large amount of substructure, consistent with a 2D fractal
distribution with dimension ~1.8 and evolve towards a uniform distribution on a
timescale of ~175 Myr. This is comparable to the crossing time of the galaxy
and we suggest that stellar structure, regardless of spatial scale, will be
eliminated in a crossing time. This may explain the smooth distribution of
stars in massive/dense young clusters in the Galaxy, while other, less massive,
clusters still display large amounts of structure at similar ages. By comparing
the stellar and star cluster distributions and evolving timescales, we show
that infant mortality of clusters (or 'popping clusters') have a negligible
influence on galactic structure. Finally, we quantify the influence of the
elongation, differential extinction, and contamination of a population on the
measured Q value.
Haas MR, Gieles M, Scheepmaker RA, Larsen SS, Lamers HJGLM, Bastian N (2006) Variation of the cluster luminosity function across the disk of M51,
We study the luminosity function (LF) of the star clusters in M51. Comparing
the observed LF with the LF resulting from artificial cluster populations
suggests that there exists an upper mass limit for clusters and that this limit
and/or the cluster disruption varies with galactocentric distance.
Gieles M (2007) Conference summary: Mass loss from stellar clusters,
This conference dealt with the mass loss from stars and from stellar
clusters. In this summary of the cluster section of the conference, I highlight
some of the results on the formation and the fundamental properties of star
clusters (Sect. 2), the early stages of their evolution (Sect. 3) and go into
more detail on the subsequent mass evolution of clusters (Sect. 4). A
discussion on how this may, or may not, depend on mass is given in Sect. 5.
Obviously, there will be a bias towards the topics where Henny Lamers has
contributed. Some of the contributions to these proceedings have already
reviewed extensively the topics of clusters mass loss and disruption, so I will
try to fit these in a general framework as much as possible.
Melo C, Downing M, Jorden P, Pasquini L, Deires S, Kelt A, Naef D, Hanuschik R, Palsa R, Castillo R, Pena E, Bendek E, Gieles M (2008) Detector upgrade for FLAMES: GIRAFFE gets red eyes, Proceedings of SPIE - The International Society for Optical Engineering 7014
GIRAFFE is an intermediate resolution spectrograph covering a wavelength range from 360-930nm and fed by optical fibers as a part of FLAMES, the multi-object fiber facility mounted at the ESO VLT Kueyen. For some time we sought a new detector for GIRAFFE spectrograph to boost the instrument's red QE (Quantum E.ciency) capabilities, while still retaining very good blue response. We aimed also at reducing the strong fringing present in the red spectra. The adopted solution was an e2v custom 2-layer AR (Anti-Re.ection) coated Deep Depletion CCD44-82 CCD. This device was made in a new e2v Technologies AR coating plant and delivered to ESO in mid 2007 with performance that matches predictions. The new CCD was commissioned in May 2008.Here we report on the results.
Gieles M (2009) Star cluster disruption, Proceedings of the International Astronomical Union
Star clusters are often used as tracers of major star formation events in
external galaxies as they can be studied up to much larger distances than
individual stars. It is vital to understand their evolution if they are used to
derive, for example, the star formation history of their host galaxy. More
specifically, we want to know how cluster lifetimes depend on their environment
and structural properties such as mass and radius. This review presents a
theoretical overview of the early evolution of star clusters and the consequent
long term survival chances. It is suggested that clusters forming with initial
densities of >10^4 Msun pc-3 survive the gas expulsion, or "infant mortality",
phase. At ~10 Myr they are bound and have densities of 10^{3+/-1} Msun pc-3.
After this time they are stable against expansion by stellar evolution,
encounters with giant molecular clouds and will most likely survive for another
Hubble time if they are in a moderate tidal field. Clusters with lower initial
densities ( Myrs. Some discussion is provided on how extra galactic star cluster
populations and especially their age distributions can be used to gain insight
in disruption.
Gieles M (2008) What determines the mass of the most massive star cluster in a galaxy:
statistics, physics or disruption?,
Astrophys.Space Sci.324:299-304,2009
In many different galactic environments the cluster initial mass function
(CIMF) is well described by a power-law with index -2. This implies a linear
relation between the mass of the most massive cluster (M_max) and the number of
clusters. Assuming a constant cluster formation rate and no disruption of the
most massive clusters it also means that M_max increases linearly with age when
determining M_max in logarithmic age bins. We observe this increase in five out
of the seven galaxies in our sample, suggesting that M_max is determined by the
size of the sample. It also means that massive clusters are very stable against
disruption, in disagreement with the mass independent disruption (MID) model
presented at this conference. For the clusters in M51 and the Antennae galaxies
the size-of-sample prediction breaks down around 10^6 M_sun, suggesting that
this is a physical upper limit to the masses of star clusters in these
galaxies. In this method there is a degeneracy between MID and a CIMF
truncation. We show how the cluster luminosity function can serve as a tool to
distinguish between the two.
Gieles M, Lamers H, Baumgardt H (2007) Star cluster life-times: dependence on mass, radius and environment,
The dissolution time (t_dis) of clusters in a tidal field does not scale with
the ``classical'' expression for the relaxation time. First, the scaling with
N, and hence cluster mass, is shallower due to the finite escape time of stars.
Secondly, the cluster half-mass radius is of little importance. This is due to
a balance between the relative tidal field strength and internal relaxation,
which have an opposite effect on t_dis, but of similar magnitude. When external
perturbations, such as encounters with giant molecular clouds (GMC) are
important, t_dis for an individual cluster depends strongly on radius. The mean
dissolution time for a population of clusters, however, scales in the same way
with mass as for the tidal field, due to the weak dependence of radius on mass.
The environmental parameters that determine t_dis are the tidal field strength
and the density of molecular gas. We compare the empirically derived t_dis of
clusters in six galaxies to theoretical predictions and argue that encounters
with GMCs are the dominant destruction mechanism. Finally, we discuss a number
of pitfalls in the derivations of t_dis from observations, such as
incompleteness, with the cluster system of the SMC as particular example.
Gieles M, Baumgardt H (2008) Lifetimes of tidally limited star clusters with different radii, MNRAS, 2008, 389, L28
We study the escape rate, dN/dt, from clusters with different radii in a
tidal field using analytical predictions and direct N-body simulations. We find
that dN/dt depends on the ratio R=r_h/r_j, where r_h is the half-mass radius
and r_j the radius of the zero-velocity surface. For R>0.05, the "tidal
regime", there is almost no dependence of dN/dt on R. To first order this is
because the fraction of escapers per relaxation time, t_rh, scales
approximately as R^1.5, which cancels out the r_h^1.5 term in t_rh. For R the "isolated regime", dN/dt scales as R^-1.5. Clusters that start with their
initial R, Ri, in the tidal regime dissolve completely in this regime and their
t_dis is insensitive to the initial r_h. We predicts that clusters that start
with Ri t_dis has a shallower dependence on Ri than what would be expected when t_dis
is a constant times t_rh. For realistic values of Ri, the lifetime varies by
less than a factor of 1.5 due to changes in Ri. This implies that the
"survival" diagram for globular clusters should allow for more small clusters
to survive. We note that with our result it is impossible to explain the
universal peaked mass function of globular cluster systems by dynamical
evolution from a power-law initial mass function, since the peak will be at
lower masses in the outer parts of galaxies. Our results finally show that in
the tidal regime t_dis scales as N^0.65/w, with w the angular frequency of the
cluster in the host galaxy. [ABRIDGED]
Silva GMD, D'Orazi V, Melo C, Torres CAO, Gieles M, Quast GR, Sterzik M (2013) Search for Associations Containing Young stars (SACY): Chemical tagging
IC 2391 & the Argus association,
Monthly Notices of the Royal Astronomical Society
We explore the possible connection between the open cluster IC 2391 and the
unbound Argus association identified by the SACY survey. In addition to common
kinematics and ages between these two systems, here we explore their chemical
abundance patterns to confirm if the two substructures shared a common origin.
We carry out a homogenous high-resolution elemental abundance study of eight
confirmed members of IC 2391 as well as six members of the Argus association
using UVES spectra. We derive spectroscopic stellar parameters and abundances
for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba.
All stars in the open cluster and Argus association were found to share
similar abundances with the scatter well within the uncertainties, where [Fe/H]
= -0.04 +/-0.03 for cluster stars and [Fe/H] = -0.06 +/-0.05 for Argus stars.
Effects of over-ionisation/excitation were seen for stars cooler than roughly
5200K as previously noted in the literature. Also, enhanced Ba abundances of
around 0.6 dex were observed in both systems. The common ages, kinematics and
chemical abundances strongly support that the Argus association stars
originated from the open cluster IC 2391. Simple modeling of this system find
this dissolution to be consistent with two-body interactions.
Sana H, de Mink SE, de Koter A, Langer N, Evans CJ, Gieles M, Gosset E, Izzard RG, Le Bouquin JB, Schneider FR (2012) Binary interaction dominates the evolution of massive stars., Science 337 (6093) pp. 444-446
The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.
Renaud F, Gieles M (2015) A flexible method to evolve collisional systems and their tidal debris
in external potentials,
We introduce a numerical method to integrate tidal effects on collisional
systems, using any definition of the external potential as a function of space
and time. Rather than using a linearisation of the tidal field, this new method
follows a differential technique to numerically evaluate the tidal acceleration
and its time derivative. Theses are then used to integrate the motions of the
components of the collisional systems, like stars in star clusters, using a
predictor-corrector scheme. The versatility of this approach allows the study
of star clusters, including their tidal tails, in complex, multi-components,
time-evolving external potentials. The method is implemented in the code nbody6
(Aarseth 2003).
Bastian N, Gieles M, Lamers HJGLM, Grijs RD, Scheepmaker RA (2004) The Star Cluster Population of M51: II. Age distribution and relations
among the derived parameters,
We use archival {\it Hubble Space Telescope} observations of broad-band
images from the ultraviolet (F255W-filter) through the near infrared (NICMOS
F160W-filter) to study the star cluster population of the interacting spiral
galaxy M51. We obtain age, mass, extinction, and effective radius estimates for
1152 star clusters in a region of $\sim 7.3 \times 8.1$ kpc centered on the
nucleus and extending into the outer spiral arms. In this paper we present the
data set and exploit it to determine the age distribution and relationships
among the fundamental parameters (i.e. age, mass, effective radius). Using this
dataset we find: {\it i}) that the cluster formation rate seems to have had a
large increase $\sim$ 50-70 Myr ago, which is coincident with the suggested
{\it second passage} of its companion, NGC 5195, {\it ii}) a large number of
extremely young ($ of unbound clusters of which a large majority will disrupt within the next
$\sim$10 Myr, and {\it iii)} that the distribution of cluster sizescan be well
approximated by a power-law with exponent, $ -\eta = -2.2 \pm 0.2$, which is
very similar to that of Galactic globular clusters, indicating that cluster
disruption is largely independent of cluster radius. In addition, we have used
this dataset to search for correlations among the derived parameters. In
particular, we do not find any strong trends between the age and mass, mass and
effective radius, nor between the galactocentric distance and effective radius.
There is, however, a strong correlation between the age of a cluster and its
extinction, with younger clusters being more heavily reddened than older
clusters.
Gieles M, Baumgardt H, Heggie D, Lamers H (2010) On the mass-radius relation of hot stellar systems, Mon. Not. R. Astron. Soc. 408, L16-L20 (2010)
Most globular clusters have half-mass radii of a few pc with no apparent
correlation with their masses. This is different from elliptical galaxies, for
which the Faber-Jackson relation suggests a strong positive correlation between
mass and radius. Objects that are somewhat in between globular clusters and
low-mass galaxies, such as ultra-compact dwarf galaxies, have a mass-radius
relation consistent with the extension of the relation for bright ellipticals.
Here we show that at an age of 10 Gyr a break in the mass-radius relation at
~10^6 Msun is established because objects below this mass, i.e. globular
clusters, have undergone expansion driven by stellar evolution and hard
binaries. From numerical simulations we find that the combined energy
production of these two effects in the core comes into balance with the flux of
energy that is conducted across the half-mass radius by relaxation. An
important property of this `balanced' evolution is that the cluster half-mass
radius is independent of its initial value and is a function of the number of
bound stars and the age only. It is therefore not possible to infer the initial
mass-radius relation of globular clusters and we can only conclude that the
present day properties are consistent with the hypothesis that all hot stellar
systems formed with the same mass-radius relation and that globular clusters
have moved away from this relation because of a Hubble time of stellar and
dynamical evolution.
Evans CJ, Bastian N, Beletsky Y, Brott I, Cantiello M, Clark JS, Crowther PA, De Koter A, De Mink SE, Dufton PL, Dunstall P, Gieles M, Gräfener G, Hénault-Brunet V, Herrero A, Howarth ID, Langer N, Lennon DJ, Maíz Apellániz J, Markova N, Najarro F, Puls J, Sana H, Simon-Díaz S, Smartt SJ, Stroud VE, Taylor WD, Trundle C, Van Loon JT, Vink JS, Walborn NR (2009) The VLT-flames tarantula survey, Proceedings of the International Astronomical Union 5 (S266) pp. 35-40
The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1000 massive stars in the 30 Doradus region in the Large Magellanic Cloud. Here, we introduce the scientific motivations of the survey and give an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution. © International Astronomical Union 2010.
Hénault-Brunet V, Evans CJ, Sana H, Gieles M, Bastian N, Apellániz JM, Markova N, Taylor WD, Bressert E, Crowther PA, Loon JTV (2012) The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the
young massive cluster R136,
Astronomy and Astrophysics
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.
Baumgardt H, Parmentier G, Gieles M, Vesperini E (2009) Evidence for two populations of Galactic globular clusters from the
ratio of their half-mass to Jacobi radii,
MON NOT R ASTRON SOC
We investigate the ratio between the half-mass radii r_h of Galactic globular
clusters and their Jacobi radii r_J given by the potential of the Milky Way and
show that clusters with galactocentric distances R_{GC}>8 kpc fall into two
distinct groups: one group of compact, tidally-underfilling clusters with
r_h/r_J r_h/r_J cluster to one of these groups and its membership in the old or younger halo
population. Based on the relaxation times and orbits of the clusters, we argue
that compact clusters and most clusters in the inner Milky Way were born
compact with half-mass radii r_h might have formed compact as well, but the majority likely formed with large
half-mass radii. Galactic globular clusters therefore show a similar dichotomy
as was recently found for globular clusters in dwarf galaxies and for young
star clusters in the Milky Way. It seems likely that some of the
tidally-filling clusters are evolving along the main sequence line of clusters
recently discovered by Kuepper et al. (2008) and are in the process of
dissolution.
Alexander P, Gieles M, Lamers H, Baumgardt H (2014) A prescription and fast code for the long-term evolution of star
clusters - III. Unequal masses and stellar evolution,
We present a new version of the fast star cluster evolution code Evolve Me A
Cluster of StarS (EMACSS). While previous versions of EMACSS reproduced
clusters of single-mass stars, this version models clusters with an evolving
stellar content. Stellar evolution dominates early evolution, and leads to: (1)
reduction of the mean mass of stars due to the mass loss of high-mass stars;
(2) expansion of the half-mass radius; (3) for (nearly) Roche Volume filling
clusters, the induced escape of stars. Once sufficient relaxation has occurred
(~ 10 relaxation times-scales), clusters reach a second, 'balanced' state
whereby the core releases energy as required by the cluster as a whole. In this
state: (1) stars escape due to tidal effects faster than before balanced
evolution; (2) the half-mass radius expands or contracts depending on the Roche
volume filling factor; and (3) the mean mass of stars increases due to the
preferential ejection of low-mass stars.
We compare the EMACSS results of several cluster properties against N-body
simulations of clusters spanning a range of initial number of stars, mass,
half-mass radius, and tidal environments, and show that our prescription
accurately predicts cluster evolution for this database. Finally, we consider
applications for EMACSS, such as studies of galactic globular cluster
populations in cosmological simulations.
Gieles M, Bastian N, Lamers H, Mout J (2005) The Star Cluster Population of M51: III. Cluster disruption and
formation history,
In this work we concentrate on the evolution of the cluster population of the
interacting galaxy M51 (NGC 5194), namely the timescale of cluster disruption
and possible variations in the cluster formation rate. We present a method to
compare observed age vs. mass number density diagrams with predicted
populations including various physical input parameters like the cluster
initial mass function, cluster disruption, cluster formation rate and star
bursts. If we assume that the cluster formation rate increases at the moments
of the encounters with NGC 5195, we find an increase in the cluster formation
rate of a factor of 3, combined with a disruption timescale which is slightly
higher then when assuming a constant formation rate (t_4 = 200 Myr vs. 100
Myr). The measured cluster disruption time is a factor of 5 shorter than
expected on theoretical grounds. This implies that the disk of M51 is not a
preferred location for survival of young globular clusters, since even clusters
with masses of the order of 10^6 M_sun will be destroyed within a few Gyr.
Sana H, Koter AD, Mink SED, Dunstall PR, Evans CJ, Henault-Brunet V, Apellaniz JM, Ramirez-Agudelo OH, Taylor WD, Walborn NR, Clark JS, Crowther PA, Herrero A, Gieles M, Langer N, Lennon DJ, Vink JS (2012) The VLT-FLAMES Tarantula Survey VIII. Multiplicity properties of the
O-type star population,
Astronomy & Physics
Aims. We analyze the multiplicity properties of the massive O-type star
population. With 360 O-type stars, this is the largest homogeneous sample of
massive stars analyzed to date.
Methods. We use multi-epoch spectroscopy and variability analysis to identify
spectroscopic binaries. We also use a Monte-Carlo method to correct for
observational biases.
Results. We observe a spectroscopic binary fraction of 0.35\pm0.03, which
corresponds to the fraction of objects displaying statistically significant
radial velocity variations with an amplitude of at least 20km/s. We compute the
intrinsic binary fraction to be 0.51\pm0.04. We adopt power-laws to describe
the intrinsic period and mass-ratio distributions: f_P ~ (log P)^\pi\ (with
0.15 power-law indexes that best reproduce the observed quantities are \pi = -0.45
+/- 0.30 and \kappa = -1.0\pm0.4. The obtained period distribution thus favours
shorter period systems compared to an Oepik law. The mass ratio distribution is
slightly skewed towards low mass ratio systems but remains incompatible with a
random sampling of a classical mass function. The binary fraction seems mostly
uniform across the field of view and independent of the spectral types and
luminosity classes. The binary fraction in the outer region of the field of
view (r > 7.8', i.e. approx117 pc) and among the O9.7 I/II objects are however
significantly lower than expected from statistical fluctuations.
Conclusions. Using simple evolutionary considerations, we estimate that over
50% of the current O star population in 30 Dor will exchange mass with its
companion within a binary system. This shows that binary interaction is greatly
affecting the evolution and fate of massive stars, and must be taken into
account to correctly interpret unresolved populations of massive stars.
Bastian N, Adamo A, Gieles M, Lamers HJGLM, Larsen SS, Silva-Villa E, Smith LJ, Kotulla R, Konstantopoulos IS, Trancho G, Zackrisson E (2011) Evidence for environmentally dependent cluster disruption in M83, Monthly Notices of the Royal Astronomical Society: Letters 417 (1)
Using multiwavelength imaging from the Wide Field Camera 3 on the Hubble Space Telescope we study the stellar cluster populations of two adjacent fields in the nearby face-on spiral galaxy, M83. The observations cover the galactic centre and reach out to ~6kpc, thereby spanning a large range of environmental conditions, ideal for testing empirical laws of cluster disruption. The clusters are selected by visual inspection to be centrally concentrated, symmetric and resolved on the images. We find that a large fraction of objects detected by automated algorithms (e.g. SExtractor or daofind) are not clusters, but rather are associations. These are likely to disperse into the field on time-scales of tens of Myr due to their lower stellar densities and not due to gas expulsion (i.e. they were never gravitationally bound). We split the sample into two discrete fields (inner and outer regions of the galaxy) and search for evidence of environmentally dependent cluster disruption. Colour-colour diagrams of the clusters, when compared to simple stellar population models, already indicate that a much larger fraction of the clusters in the outer field are older by tens of Myr than in the inner field. This impression is quantified by estimating each cluster's properties (age, mass and extinction) and comparing the age/mass distributions between the two fields. Our results are inconsistent with 'universal' age and mass distributions of clusters, and instead show that the ambient environment strongly affects the observed populations. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.
Gieles M, Moeckel N, Clarke CJ (2012) Do all stars in the solar neighbourhood form in clusters? A cautionary
note on the use of the distribution of surface densities,
Mon. Not. R. Astron. Soc. 426, L11-15 (2012)
Bressert et al. recently showed that the surface density distribution of
low-mass, young stellar objects (YSOs) in the solar neighbourhood is
approximately log-normal. The authors conclude that the star formation process
is hierarchical and that only a small fraction of stars form in dense star
clusters. Here, we show that the peak and the width of the density distribution
are also what follow if all stars form in bound clusters which are not
significantly affected by the presence of gas and expand by two-body
relaxation. The peak of the surface density distribution is simply obtained
from the typical ages (few Myr) and cluster membership number (few hundred)
typifying nearby star-forming regions. This result depends weakly on initial
cluster sizes, provided that they are sufficiently dense (initial half mass
radius of Myr. We conclude that the degeneracy of the YSO surface density distribution
complicates its use as a diagnostic of the stellar formation environment.
Hénault-Brunet V, Gieles M, Agertz O, Read JI (2015) Multiple populations in globular clusters: the distinct kinematic
imprints of different formation scenarios,
Several scenarios have been proposed to explain the presence of multiple
stellar populations in globular clusters. Many of them invoke multiple
generations of stars to explain the observed chemical abundance anomalies, but
it has also been suggested that self-enrichment could occur via accretion of
ejecta from massive stars onto the circumstellar disc of low-mass pre-main
sequence stars. These scenarios imply different initial conditions for the
kinematics of the various stellar populations. Given some net angular momentum
initially, models for which a second generation forms from gas that collects in
a cooling flow into the core of the cluster predict an initially larger
rotational amplitude for the polluted stars compared to the pristine stars.
This is opposite to what is expected from the accretion model, where the
polluted stars are the ones crossing the core and are on preferentially radial
(low-angular momentum) orbits, such that their rotational amplitude is lower.
Here we present the results of a suite of $N$-body simulations with initial
conditions chosen to capture the distinct kinematic properties of these
pollution scenarios. We show that initial differences in the kinematics of
polluted and pristine stars can survive to the present epoch in the outer parts
of a large fraction of Galactic globular clusters. The differential rotation of
pristine and polluted stars is identified as a unique kinematic signature that
could allow us to distinguish between various scenarios, while other kinematic
imprints are generally very similar from one scenario to the other.
Lamers HJGLM, Baumgardt H, Gieles M (2010) Mass loss rates and the mass evolution of star clusters, Monthly Notices of the Royal Astronomical Society
We describe the interplay between stellar evolution and dynamical mass loss
of evolving star clusters, based on the principles of stellar evolution and
cluster dynamics and on a grid of N-body simulations of cluster models. The
cluster models have different initial masses, different orbits, including
elliptical ones, and different initial density profiles. We use two sets of
cluster models: initially Roche-lobe filling and Roche-lobe underfilling. We
identify four distinct mass loss effects: (1) mass loss by stellar evolution,
(2) loss of stars induced by stellar evolution and (3) relaxation-driven mass
loss before and (4) after core collapse. Both the evolution-induced loss of
stars and the relaxation-driven mass loss need time to build up. This is
described by a delay-function of a few crossing times for Roche-lobe filling
clusters and a few half mass relaxation times for Roche-lobe underfilling
clusters. The relaxation-driven mass loss can be described by a simple power
law dependence of the mass dM/dt =-M^{1-gamma}/t0, (with M in Msun) where t0
depends on the orbit and environment of the cluster. Gamma is 0.65 for clusters
with a King-parameter W0=5 and 0.80 for more concentrated clusters with W0=7.
For initially Roche-lobe underfilling clusters the dissolution is described by
the same gamma=0.80. The values of the constant t0 are described by simple
formulae that depend on the orbit of the cluster. The mass loss rate increases
by about a factor two at core collapse and the mass dependence of the
relaxation-driven mass loss changes to gamma=0.70 after core collapse. We also
present a simple recipe for predicting the mass evolution of individual star
clusters with various metallicities and in different environments, with an
accuracy of a few percent in most cases. This can be used to predict the mass
evolution of cluster systems.
Weisz DR, Koposov SE, Dolphin AE, Belokurov V, Gieles M, Mateo ML, Olszewski EW, Sills A, Walker MG (2016) A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular
Cluster Crater,
The Astrophysical Journal: an international review of astronomy and astronomical physics 822 (1) pp. 1-10 The American Astronomical Society
We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging
taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The HST-based colormagnitude
diagram (CMD) of Crater extends 4 magnitudes below the oldest main sequence turnoff, providing
excellent leverage on Crater?s physical properties. Structurally, we find that Crater has a half-light radius of 20
pc and shows no evidence for tidal distortions. We model the CMD of Crater under the assumption of it being
a simple stellar population and alternatively by solving for its full star formation history. In both cases, Crater
is well-described by a simple stellar population with an age of 7.5 Gyr, a metallicity of [M/H] -1.65, a total
stellar mass of M? 1e4 M , a luminosity of MV -5:3, located at a distance of d 145 kpc, with modest
uncertainties in these properties due to differences in the underlying stellar evolution models. We argue that the
sparse sampling of stars above the turnoff and sub-giant branch are likely to be 1.0-1.4 M binary star systems
(blue stragglers) and their evolved descendants, as opposed to intermediate age main sequence stars. Confusion
of these populations highlights a substantial challenge in accurately characterizing sparsely populated stellar
systems. Our analysis shows that Crater is not a dwarf galaxy, but instead is an unusually young cluster given
its location in the Milky Way?s very outer stellar halo. Crater is similar to SMC cluster Lindsay 38, and its
position and velocity are in good agreement with observations and models of the Magellanic stream debris,
suggesting it may have accreted from the Magellanic Clouds. However, its age and metallicity are also in
agreement with the age-metallicity relationships of lower mass dwarf galaxies such as Leo I or Carina. Despite
uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently
than z 1 (8 Gyr ago), providing an important new constraint on the accretion history of the Milky Way.
Alexander PER, Gieles M (2012) A prescription and fast code for the long-term evolution of star
clusters,
MON NOT R ASTRON SOC
We introduce the star cluster evolution code Evolve Me A Cluster of StarS
(EMACSS), a simple yet physically motivated computational model that describes
the evolution of some fundamental properties of star clusters in static tidal
fields. We base our prescription upon the flow of energy within the cluster,
which is a constant fraction of the total energy per half-mass relaxation time.
According to Henon's predictions, this flow is independent of the precise
mechanisms for energy production within the core, and therefore does not
require a complete description of the many-body interactions therein. For a
cluster of equal-mass stars, we thence use dynamical theory and analytic
descriptions of escape mechanisms to construct a series of coupled differential
equations expressing the time evolution of cluster mass and radius. These
equations are numerically solved using a fourth-order Runge-Kutta integration
kernel, and the results benchmarked against a data base of direct N-body
simulations. We use simulations containing a modest initial number of stars
(1024 prescription is publicly available and reproduces the N-body results to within
~10 per cent accuracy for the entire post-collapse evolution of star clusters.
Evans CJ, Taylor WD, Henault-Brunet V, Sana H, Koter AD, Simon-Diaz S, Carraro G, Bagnoli T, Bastian N, Bestenlehner JM, Bonanos AZ, Bressert E, Brott I, Campbell MA, Cantiello M, Clark JS, Costa E, Crowther PA, Mink SED, Doran E, Dufton PL, Dunstall PR, Friedrich K, Garcia M, Gieles M, Graefener G, Herrero A, Howarth ID, Izzard RG, Langer N, Lennon DJ, Apellaniz JM, Markova N, Najarro F, Puls J, Ramirez OH, Sabin-Sanjulian C, Smartt SJ, Stroud VE, Loon JTV, Vink JS, Walborn NR (2011) The VLT-FLAMES Tarantula Survey I: Introduction and observational
overview,
Astronomy and Astrophysics 530
The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has
obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30
Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our
scientific motivations and give an overview of the survey targets, including
optical and near-infrared photometry and comprehensive details of the data
reduction. One of the principal objectives was to detect massive binary systems
via variations in their radial velocities, thus shaping the multi-epoch
observing strategy. Spectral classifications are given for the massive
emission-line stars observed by the survey, including the discovery of a new
Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To
illustrate the diversity of objects encompassed by the survey, we investigate
the spectral properties of sixteen targets identified by Gruendl & Chu from
Spitzer photometry as candidate young stellar objects or stars with notable
mid-infrared excesses. Detailed spectral classification and quantitative
analysis of the O- and B-type stars in the VFTS sample, paying particular
attention to the effects of rotational mixing and binarity, will be presented
in a series of future articles to address fundamental questions in both stellar
and cluster evolution.
Evans CJ, Bastian N, Beletsky Y, Brott I, Cantiello M, Clark JS, Crowther PA, Koter AD, Mink SD, Dufton PL, Dunstall P, Gieles M, Graefener G, Henault-Brunet V, Herrero A, Howarth ID, Langer N, Lennon DJ, Apellaniz JM, Markova N, Najarro F, Puls J, Sana H, Simon-Diaz S, Smartt SJ, Stroud VE, Taylor WD, Trundle C, Loon JTV, Vink JS, Walborn NR (2009) The VLT-FLAMES Tarantula Survey,
The Tarantula Survey is an ambitious ESO Large Programme that has obtained
multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region
of the Large Magellanic Cloud. Here we introduce the scientific motivations of
the survey and give an overview of the observational sample. Ultimately,
quantitative analysis of every star, paying particular attention to the effects
of rotational mixing and binarity, will be used to address fundamental
questions in both stellar and cluster evolution.
Davies B, Bastian N, Gieles M, Seth AC, Mengel S, Konstantopoulos IS (2010) GLIMPSE-CO1: the most massive intermediate-age stellar cluster in the
Galaxy,
Monthly Notices of the Royal Astronomical Society
The stellar cluster GLIMPSE-C01 is a dense stellar system located in the
Galactic Plane. Though often referred to in the literature as an old globular
cluster traversing the Galactic disk, previous observations do not rule out
that it is an intermediate age (less than a few Gyr) disk-borne cluster. Here,
we present high-resolution near-infrared spectroscopy of over 50 stars in the
cluster. We find an average radial velocity is consistent with being part of
the disk, and determine the cluster's dynamical mass to be (8 \pm 3)x10^4 Msun.
Analysis of the cluster's M/L ratio, the location of the Red Clump, and an
extremely high stellar density, all suggest an age of 400-800Myr for
GLIMPSE-C01, much lower than for a typical globular cluster. This evidence
therefore leads us to conclude that GLIMPSE-C01 is part of the disk population,
and is the most massive Galactic intermediate-age cluster discovered to date.
Gieles M, Lamers HJGLM, Zwart SFP (2007) On the Interpretation of the Age Distribution of Star Clusters in the
Small Magellanic Cloud,
We re-analyze the age distribution (dN/dt) of star clusters in the Small
Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud
Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can
be approximated by a power-law distribution, dN/dt propto t^-beta, with
-beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive
the ages. Predictions for a cluster population without dissolution limited by a
V-band detection result in a power-law dN/dt distribution with an index of
~-0.7. This is because the limiting cluster mass increases with age, due to
evolutionary fading of clusters, reducing the number of observed clusters at
old ages. When a mass cut well above the limiting cluster mass is applied, the
dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is
of small importance in shaping the dN/dt distribution and incompleteness causes
dN/dt to decline. The reason that no (mass independent) infant mortality of
star clusters in the first ~10-20 Myr is found is explained by a detection bias
towards clusters without nebular emission, i.e. cluster that have survived the
infant mortality phase. The reason we find no evidence for tidal (mass
dependent) cluster dissolution in the first Gyr is explained by the weak tidal
field of the SMC. Our results are in sharp contrast to the interpretation of
Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid
cluster dissolution. This is due to their erroneous assumption that the sample
is limited by cluster mass, rather than luminosity.
Konstantopoulos IS, Bastian N, Gieles M, Lamers HJGLM (2009) Constraining star cluster disruption mechanisms, 2010IAUS..266..433K
Star clusters are found in all sorts of environments and their formation and
evolution is inextricably linked to the star formation process. Their eventual
destruction can result from a number of factors at different times, but the
process can be investigated as a whole through the study of the cluster age
distribution. Observations of populous cluster samples reveal a distribution
following a power law of index approximately -1. In this work we use M33 as a
test case to examine the age distribution of an archetypal cluster population
and show that it is in fact the evolving shape of the mass detection limit that
defines this trend. That is to say, any magnitude-limited sample will appear to
follow a dN/dt=1/t, while cutting the sample according to mass gives rise to a
composite structure, perhaps implying a dependence of the cluster disruption
process on mass. In the context of this framework, we examine different models
of cluster disruption from both theoretical and observational standpoints.
Sana H, Momany Y, Gieles M, Carraro G, Beletsky Y, Ivanov VD, Silva GD, James G (2010) A MAD view of Trumpler 14, Astronomy and Astrophysics
We present adaptive optics (AO) near-infrared observations of the core of the
Tr 14 cluster in the Carina region obtained with the ESO multi-conjugate AO
demonstrator, MAD. Our campaign yields AO-corrected observations with an image
quality of about 0.2 arcsec across the 2 arcmin field of view, which is the
widest AO mosaic ever obtained. We detected almost 2000 sources spanning a
dynamic range of 10 mag. The pre-main sequence (PMS) locus in the
colour-magnitude diagram is well reproduced by Palla & Stahler isochrones with
an age of 3 to 5 1E+05 yr, confirming the very young age of the cluster. We
derive a very high (deprojected) central density n0~4.5(+/-0.5) \times 10^4
pc^-3 and estimate the total mass of the cluster to be about ~4.3^{+3.3}_{-1.5}
\times 10^3 Msun, although contamination of the field of view might have a
significant impact on the derived mass. We show that the pairing process is
largely dominated by chance alignment so that physical pairs are difficult to
disentangle from spurious ones based on our single epoch observation. Yet, we
identify 150 likely bound pairs, 30% of these with a separation smaller than
0.5 arcsec (~1300AU). We further show that at the 2-sigma level massive stars
have more companions than lower-mass stars and that those companions are
respectively brighter on average, thus more massive. Finally, we find some
hints of mass segregation for stars heavier than about 10 Msun. If confirmed,
the observed degree of mass segregation could be explained by dynamical
evolution, despite the young age of the cluster.
Bastian N, Ercolano B, Gieles M, Rosolowsky E, Scheepmaker RA, Gutermuth R, Efremov Y (2007) Hierarchical Star-Formation in M33: Fundamental properties of the
star-forming regions,
Mon.Not.Roy.Astron.Soc. 379 pp. 1302-1312
Star-formation within galaxies appears on multiple scales, from spiral
structure, to OB associations, to individual star clusters, and often
sub-structure within these clusters. This multitude of scales calls for
objective methods to find and classify star-forming regions, regardless of
spatial size. To this end, we present an analysis of star-forming groups in the
local group spiral galaxy M33, based on a new implementation of the Minimum
Spanning Tree (MST) method. Unlike previous studies which limited themselves to
a single spatial scale, we study star-forming structures from the effective
resolution limit (~20pc) to kpc scales. We find evidence for a continuum of
star-forming group sizes, from pc to kpc scales. We do not find a
characteristic scale for OB associations, unlike that found in previous
studies, and we suggest that the appearance of such a scale was caused by
spatial resolution and selection effects. The luminosity function of the groups
is found to be well represented by a power-law with an index, -2, similar to
that found for clusters and GMCs. Additionally, the groups follow a similar
mass-radius relation as GMCs. The size distribution of the groups is best
described by a log-normal distribution and we show that within a hierarchical
distribution, if a scale is selected to find structure, the resulting size
distribution will have a log-normal distribution. We find an abrupt drop of the
number of groups outside a galactic radius of ~4kpc, suggesting a change in the
structure of the star-forming ISM, possibly reflected in the lack of GMCs
beyond this radius. (abridged)
Lamers H, Gieles M, Bastian N, Baumgardt H, Kharchenko N, Zwart SP (2005) An analytical description of the disruption of star clusters in tidal
fields with an application to Galactic open clusters,
We present a simple analytical description of the disruption of star clusters
in a tidal field, which agrees excellently with detailed N-body simulations.
The analytic expression can be used to predict the mass and age histograms of
surviving clusters for any cluster initial mass function and any cluster
formation history. The method is applied to open clusters in the solar
neighbourhood, based on the new cluster sample of Kharchenko et al. From a
comparison between the observed and predicted age distributions in the age
range between 10 Myr to 3 Gyr we find the following results: (1) The disruption
time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr.
This is a factor 5 shorter than derived from N-body simulations of clusters in
the tidal field of the galaxy. (2) The present starformation rate in bound
clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which
corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7
10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump
between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher
than before and after. (4) The present star formation rate in bound clusters is
half as small as that derived from the study of embedded clusters. The
difference suggests that half of the clusters in the solar neighbourhood become
unbound within 10 Myr. (5) The most massive clusters within 600 pc had an
initial mass of 3*10^4 M_sun. This is in agreement with the statistically
expected value based on a cluster initial mass function with a slope of -2,
even if the physical upper mass limit is as high as 10^6 M_sun.
Scheepmaker RA, Haas MR, Gieles M, Bastian N, Larsen SS, Lamers HJGLM (2007) ACS imaging of star clusters in M51. I. Identification and radius
distribution,
We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and
F814W to select a large sample of star clusters with accurate effective radius
measurements in an area covering the complete disc of M51. We present the
dataset and study the radius distribution and relations between radius, colour,
arm/interarm region, galactocentric distance, mass and age. We select a sample
of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and
derive their effective radii by fitting the spatial profiles with analytical
models convolved with the point spread function. The radii of 1284 clusters are
studied in detail. We find cluster radii between 0.5 and ~10 pc, and one
exceptionally large cluster candidate with a radius of 21.6 pc. The median
radius is 2.1 pc. We find 70 clusters in our sample which have colours
consistent with being old GC candidates and we find 6 new "faint fuzzy"
clusters in, or projected onto, the disc of M51. The radius distribution can
not be fitted with a power law, but a log-normal distribution provides a
reasonable fit to the data. This indicates that shortly after the formation of
the clusters from a fractal gas, their radii have changed in a non-uniform way.
We find an increase in radius with colour as well as a higher fraction of
redder clusters in the interarm regions, suggesting that clusters in spiral
arms are more compact. We find a correlation between radius and galactocentric
distance which is considerably weaker than the observed correlation for old
Milky Way GCs. We find weak relations between cluster luminosity and radius,
but we do not observe a correlation between cluster mass and radius.
Cabrera-Ziri I, Niederhofer F, Bastian N, Rejkuba M, Balbinot E, Kerzendorf WE, Larsen SS, Mackey AD, Dalessandro E, Mucciarelli A, Charbonnel C, Hilker M, Gieles M, Henault-Brunet V (2016) No evidence for younger stellar generations within the intermediate-age massive clusters NGC 1783, NGC 1806 and NGC 411, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 459 (4) pp. 4218-4223 OXFORD UNIV PRESS
Shanahan RL, Gieles M (2015) Biases in the inferred mass-to-light ratio of globular clusters: no need
for variations in the stellar mass function,
Mon. Not. R. Astron. Soc. 448 pp. 94-98
From a study of the integrated light properties of 200 globular clusters
(GCs) in M31, Strader et al. found that the mass-to-light ratios are lower than
what is expected from simple stellar population (SSP) models with a `canonical'
stellar initial mass function (IMF), with the discrepancy being larger at high
metallicities. We use dynamical multi-mass models, that include a prescription
for equipartition, to quantify the bias in the inferred dynamical mass as the
result of the assumption that light follows mass. For a universal IMF and a
metallicity dependent present day mass function we find that the inferred mass
from integrated light properties systematically under estimates the true mass,
and that the bias is more important at high metallicities, as was found for the
M31 GCs. We show that mass segregation and a flattening of the mass function
have opposing effects of similar magnitude on the mass inferred from integrated
properties. This makes the mass-to-light ratio as derived from integrated
properties an inadequate probe of the low-mass end of the stellar mass
function. There is, therefore, no need for variations in the IMF, nor the need
to invoke depletion of low-mass stars, to explain the observations. Finally, we
find that the retention fraction of stellar-mass black holes (BHs) is an
equally important parameter in understanding the mass segregation bias. We
speculatively put forward to idea that kinematical data of GCs can in fact be
used to constrain the total mass in stellar-mass BHs in GCs.
Gieles M, Heggie DC, Zhao H (2011) The life cycle of star cluster in a tidal field, Mon. Not. R. Astron. Soc. 413, 2509-2524 (2011)
The evolution of globular clusters due to 2-body relaxation results in an
outward flow of energy and at some stage all clusters need a central energy
source to sustain their evolution. Henon provided the insight that we do not
need to know the details of the energy production in order to understand the
relaxation-driven evolution of the cluster, at least outside the core. He
provided two self-similar solutions for the evolution of clusters based on the
view that the cluster as a whole determines the amount of energy that is
produced in the core: steady expansion for isolated clusters, and homologous
contraction for clusters evaporating in a tidal field. We combine these models:
the half-mass radius increases during the first half of the evolution, and
decreases in the second half; while the escape rate approaches a constant value
set by the tidal field. We refer to these phases as `expansion dominated' and
`evaporation dominated'. These simple analytical solutions immediately allow us
to construct evolutionary tracks and isochrones in terms of cluster half-mass
density, cluster mass and galacto-centric radius. From a comparison to the
Milky Way globular clusters we find that roughly 1/3 of them are in the second,
evaporation-dominated phase and for these clusters the density inside the
half-mass radius varies with the galactocentric distance R as rho_h ~ 1/R^2.
The remaining 2/3 are still in the first, expansion-dominated phase and their
isochrones follow the environment-independent scaling rho_h ~ M^2; that is, a
constant relaxation time-scale. We find substantial agreement between Milky Way
globular cluster parameters and the isochrones, which suggests that there is,
as Henon suggested, a balance between the flow of energy and the central energy
production for almost all globular clusters.
Haas MR, Gieles M, Scheepmaker RA, Larsen SS, Lamers HJGLM (2008) ACS imaging of star clusters in M51 II. The luminosity function and mass
function across the disk,
Astronomy and Astrophysics, Volume 487, Issue 3, 2008, pp.937-949
Whether or not there exists a physical upper mass limit for star clusters is
still unclear. HST/ACS data for the rich cluster population in the interacting
galaxy M51 enables us to investigate this in more detail. We investigate
whether the cluster luminosity function (LF) in M51 shows evidence for an upper
limit to the mass function. The variations of the LF parameters with position
on the disk are addressed. We determine the cluster LF for all clusters in M51
falling within our selection criteria, as well as for several subsets of the
sample. In that way we can determine the properties of the cluster population
as a function of galactocentric distance and background intensity. By comparing
observed and simulated LFs we can constrain the underlying cluster initial mass
function and/or cluster disruption parameters. A physical upper mass limit for
star clusters will appear as a bend dividing two power law parts in the LF, if
the cluster sample is large enough to sample the full range of cluster masses.
The location of the bend in the LF is indicative of the value of the upper mass
limit. The slopes of the power laws are an interplay between upper masses,
disruption and fading. The LF of the cluster population of M51 is better
described by a double power law than by a single power law. We show that the
cluster initial mass function is likely to be truncated at the high mass end.
We conclude from the variation of the LF parameters with galactocentric
distance that both the upper mass limit and the cluster disruption parameters
are likely to be a function of position in the galactic disk. At higher
galactocentric distances the maximum mass is lower, cluster disruption slower,
or both.
Markova N, Evans C, Bastian N, Beletsky Y, Bestenlehner J, Brott I, Cantiello M, Carraro G, Clark J, Crowther P, Koter AD, Mink SD, Doran E, Dufton P, Dunstall P, Gieles M, Graefener G, Henault-Brunet V, Herrero A, Howarth I, Langer N, Lennon D, Apellaniz JM, Najarro F, Puls J, Sana H, Simon-Diaz S, Smartt S, Stroud V, Taylor W, Loon JV, Vink J, Walborn N (2011) The FLAMES Tarantula Survey, Astronomy and Astrophysics
The Tarantula survey is an ESO Large Programme which has obtained
multi-epochs spectroscopy of over 800 massive stars in the 30 Dor region in the
Large Magelanic Cloud. Here we briefly describe the main drivers of the survey
and the observational material derived.
Gieles M, Bastian N (2008) An alternative method to study star cluster disruption, ASTRON ASTROPHYS
Many embedded star clusters do not evolve into long-lived bound clusters. The
most popular explanation for this "infant mortality" of young clusters is the
expulsion of natal gas by stellar winds and supernovae, which leaves up to 90%
of them unbound. A cluster disruption model has recently been proposed in which
this mass- independent disruption of clusters proceeds for another Gyr after
gas expulsion. In this scenario, the survival chances of massive clusters are
much smaller than in the traditional mass-dependent disruption models. The most
common way to study cluster disruption is to use the cluster age distribution,
which, however, can be heavily affected by incompleteness. To avoid this, we
introduce a new method, based on size-of-sample effects, namely the relation
between the most massive cluster, M_max, and the age range sampled. Assuming
that clusters are sampled from a power-law initial mass function, with index -2
and that the cluster formation rate is constant, M_max scales with the age
range sampled, such that the slope in a log(M_max) vs. log(age) plot is equal
to unity. This slope decreases if mass-independent disruption is included. For
90% mass-independent cluster disruption per age dex, the predicted slope is
zero. For the solar neighbourhood, SMC, LMC, M33, and M83, based on ages and
masses taken from the literature, we find slopes consistent with the expected
size-of-sample correlations for the first 100 Myr, hence ruling out the 90%
mass-independent cluster disruption scenario. For M51, however, the increase of
log(M_max) with log(age) is slightly shallower and for the Antennae galaxies it
is flat. This simple method shows that the formation and/or disruption of
clusters in the Antennae must have been very different from that of the other
galaxies studied here, so it should not be taken as a representative case.
Hénault-Brunet V, Gieles M, Evans CJ, Sana H, Bastian N, Apellániz JM, Taylor WD, Markova N, Bressert E, Koter AD, Loon JTV (2012) The VLT-FLAMES Tarantula Survey VI: Evidence for rotation of the young
massive cluster R136,
Astronomy and Astrophysics
Although it has important ramifications for both the formation of star
clusters and their subsequent dynamical evolution, rotation remains a largely
unexplored characteristic of young star clusters (few Myr). Using multi-epoch
spectroscopic data of the inner regions of 30 Doradus in the Large Magellanic
Cloud (LMC) obtained as part of the VLT-FLAMES Tarantula Survey, we search for
rotation of the young massive cluster R136. From the radial velocities of 36
apparently single O-type stars within a projected radius of 10 pc from the
centre of the cluster, we find evidence, at the 95% confidence level, for
rotation of the cluster as a whole. We use a maximum likelihood method to fit
simple rotation curves to our data and find a typical rotational velocity of ~3
km/s. When compared to the low velocity dispersion of R136, our result suggests
that star clusters may form with at least ~20% of the kinetic energy in
rotation.
Gaburov E, Gieles M (2008) Mass segregation in young star clusters: can it be detected from the
integrated photometric properties?,
Monthly Notices of the Royal Astronomical Society
We consider the effect of mass segregation on the observable integrated
properties of star clusters. The measurable properties depend on a combination
of the dynamical age of the cluster and the physical age of the stars in the
cluster. To investigate all possible combinations of these two quantities we
propose an analytical model for the mass function of segregated star clusters
that agrees with the results of N-body simulations, in which any combination
can be specified. For a realistic degree of mass segregation and a fixed
density profile we find with increasing age an increase in the measured core
radii and a central surface brightness that decreases in all filters more
rapidly than what is expected from stellar evolution alone. Within a Gyr the
measured core radius increases by a factor of two and the central surface
density in all filters of a segregated cluster will be overestimated by a
similar factor when not taking into account mass segregation in the conversion
from light to mass. We find that the $V-I$ colour of mass segregated clusters
decreases with radius by about 0.1-0.2 mag, which could be observable. From
recent observations of partially resolved extra-galactic clusters a decreasing
half-light radius with increasing wavelength was observed, which was attributed
to mass segregation. These observations can not be reproduced by our models. We
find that the differences between measured radii in different filters are
always smaller than 5%.
Campbell MA, Evans CJ, Ascenso J, Longmore AJ, Kolb J, Gieles M, Alves J (2008) Imaging the dense stellar cluster R136 with VLT-MAD, Proceedings of SPIE
We evaluate the performance of the Multi-conjugate Adaptive optics
Demonstrator (MAD) from H and Ks imaging of 30 Doradus in the Large Magellanic
Cloud. Maps of the full-width half maximum (FWHM) of point sources in the H and
Ks images are presented, together with maps of the Strehl ratio achieved in the
Ks-band observations. Each of the three natural guide stars was at the edge of
the MAD field-of-view, and the observations were obtained at relatively large
airmass (1.4-1.6). Even so, the Strehl ratio achieved in the second pointing
(best-placed compared to the reference stars) ranged from 15% to an impressive
30%. Preliminary photometric calibration of the first pointing indicates 5
sigma sensitivities of Ks=21.75 and H=22.25 (from 22 and 12 min exposures,
respectively).
Gieles M, Lamers HJGLM, Baumgardt H (2007) Star cluster life-times: Dependence on mass, radius and environment, Proceedings of the International Astronomical Union 3 (S246) pp. 171-175
The dissolution time (tdis) of clusters in a tidal field does not scale with the classical expression for the relaxation time. First, the scaling with N, and hence cluster mass, is shallower due to the finite escape time of stars. Secondly, the cluster half-mass radius is of little importance. This is due to a balance between the relative tidal field strength and internal relaxation, which have an opposite effect on tdis, but of similar magnitude. When external perturbations, such as encounters with giant molecular clouds (GMC) are important, tdis for an individual cluster depends strongly on radius. The mean dissolution time for a population of clusters, however, scales in the same way with mass as for the tidal field, due to the weak dependence of radius on mass. The environmental parameters that determine tdis are the tidal field strength and the density of molecular gas. We compare the empirically derived tdis of clusters in six galaxies to theoretical predictions and argue that encounters with GMCs are the dominant destruction mechanism. Finally, we discuss a number of pitfalls in the derivations of tdis from observations, such as incompleteness, with the cluster system of the SMC as particular example. © 2008 Copyright International Astronomical Union 2008.
Lamers HJGLM, Gieles M (2007) Star clusters in the solar neighborhood: a solution to Oort's problem,
In 1958 Jan Oort remarked that the lack of old clusters in the solar
neighborhood (SN) implies that clusters are destroyed on a timescale of less
than a Gyr. This is much shorter than the predicted dissolution time of
clusters due to stellar evolution and two-body relaxation in the tidal field of
the Galaxy. So, other (external) effects must play a dominant role in the
destruction of star clusters in the solar neighborhood. We recalculated the
survival time of initially bound star clusters in the solar neighborhood taking
into account: (1) stellar evolution, (2) tidal stripping, (3) perturbations by
spiral arms and (4) encounters with giant molecular clouds (GMCs). We find that
encounters with GMCs are the most damaging to clusters. The resulting predicted
dissolution time of these combined effects, t_dis=1.7 (Mi/10^4 M_sun)^0.67 Gyr
for clusters in the mass range of 10^2 disruption time of t_dis=1.3+/-0.5 (M/10^4 M_sun)^0.62 Gyr that was derived
empirically from a mass limited sample of clusters in the solar neighborhood
within 600 pc. The predicted shape of the age distribution of clusters agrees
very well with the observed one. The comparison between observations and theory
implies a surface star formation rate (SFR) near the sun of 3.5x10^-10 M_sun
yr^-1 pc^-2 for stars in bound clusters with an initial mass in the range of
10^2 to 3x10^4 M_sun. This can be compared to a total SFR of 7-10x10^-10 M_sun
yr^-1 pc^-2 derived from embedded clusters or 3-7x10^-9 M_sun yr^-1 pc^-2
derived from field stars. This implies an infant mortality rate of clusters in
the solar neighborhood between 50% and 95%, in agreement with the results of a
study of embedded clusters.
Gieles M, Bastian N, Ercolano B (2008) Evolution of stellar structure in the Small Magellanic Cloud, Mon.Not.Roy.Astron.Soc.391:L93-L97,2008
The projected distribution of stars in the Small Magellanic Cloud (SMC) from
the Magellanic Clouds Photometric Survey is analysed. Stars of different ages
are selected via criteria based on V magnitude and V-I colour, and the degree
of `grouping' as a function of age is studied. We quantify the degree of
structure using the two-point correlation function and a method based on the
Minimum Spanning Tree and find that the overall structure of the SMC is
evolving from a high degree of sub-structure at young ages (~10 Myr) to a
smooth radial density profile. This transition is gradual and at ~75 Myr the
distribution is statistically indistinguishable from the background SMC
distribution. This time-scale corresponds to approximately the dynamical
crossing time of stars in the SMC. The spatial positions of the star clusters
in the SMC show a similar evolution of spatial distribution with age. Our
analysis suggests that stars form with a high degree of (fractal)
sub-structure, probably imprinted by the turbulent nature of the gas from which
they form, which is erased by random motions in the galactic potential on a
time-scale of a galactic crossing time.
Bastian N, Gieles M, Goodwin SP, Trancho G, Smith LJ, Konstantopoulos I, Efremov Y (2008) The Early Expansion of Cluster Cores, Monthly Notices of the Royal Astronomical Society
The observed properties of young star clusters, such as the core radius and
luminosity profile, change rapidly during the early evolution of the clusters.
Here we present observations of 6 young clusters in M51 where we derive their
sizes using HST imaging and ages using deep Gemini-North spectroscopy. We find
evidence for a rapid expansion of the cluster cores during the first 20 Myr of
their evolution. We confirm this trend by including data from the literature of
both Galactic and extra-galactic embedded and young clusters, and possible
mechanisms (rapid gas removal, stellar evolutionary mass-loss, and internal
dynamical heating) are discussed. We explore the implications of this result,
focussing on the fact that clusters were more concentrated in the past,
implying that their stellar densities were much higher and relaxation times
correspondingly shorter. Thus, when estimating if a particular cluster is
dynamically relaxed, (i.e. when determining if a cluster's mass segregation is
due to primordial or dynamical processes), the current relaxation time is only
an upper-limit, with the relaxation time likely being significantly shorter in
the past.
Gieles M, Zwart SP (2010) The distinction between star clusters and associations, Mon. Not. R. Astron. Soc. 410, L6-L7 (2011)
In Galactic studies a distinction is made between (open) star clusters and
associations. For barely resolved objects at a distance of several Mpc this
distinction is not trivial to make. Here we provide an objective definition by
comparing the age of the stars to the crossing time of nearby stellar
agglomerates. We find that a satisfactory separation can be made where this
ratio equals unity. Stellar agglomerates for which the age of the stars exceeds
the crossing time are bound, and are referred to as star clusters.
Alternatively, those for which the crossing time exceeds the stellar age are
unbound and are referred to as associations. This definition is useful whenever
reliable measurements for the mass, radius and age are available.
Niederste-Ostholt M, Belokurov V, Evans NW, Koposov S, Gieles M, Irwin MJ (2010) The tidal tails of the ultrafaint globular cluster Palomar 1, Monthly Notices of the Royal Astronomical Society: Letters 408 (1)
Using the optimal filter technique applied to Sloan Digital Sky Survey photometry, we have found extended tails stretching about 1° (or several tens of half-light radii) from either side of the ultrafaint globular cluster Palomar 1. The tails contain roughly as many stars as does the cluster itself. Using deeper Hubble Space Telescope data, we see that the isophotes twist in a characteristic S-shape on moving outwards from the cluster centre to the tails. We argue that the main mechanism forming the tails may be relaxation-driven evaporation and that Pal 1 may have been accreted from a now disrupted dwarf galaxy ~500 Myr ago. © 2010 The Authors. Journal compilation © 2010 RAS.
Koposov SE, Belokurov V, Evans NW, Gilmore G, Gieles M, Irwin MJ, Lewis GF, Niederste-Ostholt M, Penarrubia J, Smith MC, Bizyaev D, Malanushenko E, Malanushenko V, Schneider DP, Wyse RFG (2011) The Sagittarius Streams in the Southern Galactic Hemisphere, Astrophysical Journal
The structure of the Sagittarius stream in the Southern Galactic hemisphere
is analysed with the Sloan Digital Sky Survey Data Release 8. Parallel to the
Sagittarius tidal track, but ~ 10deg away, there is another fainter and more
metal-poor stream. We provide evidence that the two streams follow similar
distance gradients but have distinct morphological properties and stellar
populations. The brighter stream is broader, contains more metal-rich stars and
has a richer colour-magnitude diagram with multiple turn-offs and a prominent
red clump as compared to the fainter stream. Based on the structural properties
and the stellar population mix, the stream configuration is similar to the
Northern "bifurcation". In the region of the South Galactic Cap, there is
overlapping tidal debris from the Cetus Stream, which crosses the Sagittarius
stream. Using both photometric and spectroscopic data, we show that the blue
straggler population belongs mainly to Sagittarius and the blue horizontal
branch stars belong mainly to the Cetus stream in this confused location in the
halo.
Gieles M, Sana H, Zwart SFP (2009) On the velocity dispersion of young star clusters: super-virial or
binaries?,
Monthly Notices of the Royal Astronomical Society
Many young extra-galactic clusters have a measured velocity dispersion that
is too high for the mass derived from their age and total luminosity, which has
led to the suggestion that they are not in virial equilibrium. Most of these
clusters are confined to a narrow age range centred around 10 Myr because of
observational constraints. At this age the cluster light is dominated by
luminous evolved stars, such as red supergiants, with initial masses of ~13-22
Msun for which (primordial) binarity is high. In this study we investigate to
what extent the observed excess velocity dispersion is the result of the
orbital motions of binaries. We demonstrate that estimates for the dynamical
mass of young star clusters, derived from the observed velocity dispersion,
exceed the photometric mass by up-to a factor of 10 and are consistent with a
constant offset in the square of the velocity dispersion. This can be
reproduced by models of virialised star clusters hosting a massive star
population of which ~25 is in binaries, with typical mass ratios of ~0.6 and
periods of ~1000 days. We conclude that binaries play a pivotal role in
deriving the dynamical masses of young (~10 Myr) moderately massive and compact
( 1 pc) star clusters.
Renaud F, Gieles M, Boily C (2011) Evolution of star clusters in arbitrary tidal fields, Monthly Notices of the Royal Astronomical Society
We present a novel and flexible tensor approach to computing the effect of a
time-dependent tidal field acting on a stellar system. The tidal forces are
recovered from the tensor by polynomial interpolation in time. The method has
been implemented in a direct-summation stellar dynamics integrator (NBODY6) and
test-proved through a set of reference calculations: heating, dissolution time
and structural evolution of model star clusters are all recovered accurately.
The tensor method is applicable to arbitrary configurations, including the
important situation where the background potential is a strong function of
time. This opens up new perspectives in stellar population studies reaching to
the formation epoch of the host galaxy or galaxy cluster, as well as for
star-burst events taking place during the merger of large galaxies. A pilot
application to a star cluster in the merging galaxies NGC 4038/39 (the
Antennae) is presented.
Zocchi A, Gieles M, Henault-Brunet V, Varri AL (2016) Testing lowered isothermal models with direct N-body simulations of globular clusters, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 462 (1) pp. 696-714 OXFORD UNIV PRESS
Gieles M, Athanassoula E, Zwart SFP (2007) The effect of spiral arm passages on the evolution of stellar clusters, Mon.Not.Roy.Astron.Soc. 376 pp. 809-819
We study the effect of spiral arm passages on the evolution of star clusters
on planar and circular orbits around the centres of galaxies. Individual
passages with different relative velocity (V_drift) and arm width are studied
using N-body simulations. When the ratio of the time it takes the cluster to
cross the density wave to the crossing time of stars in the cluster is much
smaller than one, the energy gain of stars can be predicted accurately in the
impulsive approximation. When this ratio is much larger than one, the cluster
is heated adiabatically and the net effect of heating is largely damped. For a
given duration of the perturbation, this ratio is smaller for stars in the
outer parts of the cluster compared to stars in the inner part. The cluster
energy gain due to perturbations of various duration as obtained from our
N-body simulations is in good agreement with theoretical predictions taking
into account the effect of adiabatic damping. Perturbations by the broad
stellar component of the spiral arms on a cluster are in the adiabatic regime
and, therefore, hardly contribute to the energy gain and mass loss of the
cluster. We consider the effect of crossings through the high density shocked
gas in the spiral arms, which result in a more impulsive compression of the
cluster. The time scale of disruption is shortest at ~0.8-0.9 R_CR since there
V_drift is low. This location can be applicable to the solar neighbourhood. In
addition, the four-armed spiral pattern of the Milky Way makes spiral arms
contribute more to the disruption of clusters than in a similar but two-armed
galaxy. Still, the disruption time due to spiral arm perturbations there is
about an order of magnitude higher than what is observed for the solar
neighbourhood.[ABRIDGED]
Sollima A, Baumgardt H, Zocchi A, Balbinot E, Gieles M, Henault-Brunet V, Varri AL (2015) Biases in the determination of dynamical parameters of star clusters:
today and in the Gaia era,
The structural and dynamical properties of star clusters are generally
derived by means of the comparison between steady-state analytic models and the
available observables. With the aim of studying the biases of this approach, we
fitted different analytic models to simulated observations obtained from a
suite of direct N-body simulations of star clusters in different stages of
their evolution and under different levels of tidal stress to derive mass, mass
function and degree of anisotropy. We find that masses can be
under/over-estimated up to 50% depending on the degree of relaxation reached by
the cluster, the available range of observed masses and distances of radial
velocity measures from the cluster center and the strength of the tidal field.
The mass function slope appears to be better constrainable and less sensitive
to model inadequacies unless strongly dynamically evolved clusters and a
non-optimal location of the measured luminosity function are considered. The
degree and the characteristics of the anisotropy developed in the N-body
simulations are not adequately reproduced by popular analytic models and can be
detected only if accurate proper motions are available. We show how to reduce
the uncertainties in the mass, mass-function and anisotropy estimation and
provide predictions for the improvements expected when Gaia proper motions will
be available in the near future.
Zocchi A, Gieles M, Hénault-Brunet V (2015) On the uniqueness of kinematical signatures of intermediate-mass black
holes in globular clusters,
Finding an intermediate-mass black hole (IMBH) in a globular cluster (GC), or
proving its absence, is a crucial ingredient in our understanding of galaxy
formation and evolution. The challenge is to identify a unique signature of an
IMBH that cannot be accounted for by other processes. Observational claims of
IMBH detection are often based on analyses of the kinematics of stars, such as
a rise in the velocity dispersion profile towards the centre. In this
contribution we discuss the degeneracy between this IMBH signal and pressure
anisotropy in the GC. We show that that by considering anisotropic models it is
possible to partially explain the innermost shape of the projected velocity
dispersion profile, even though models that do not account for an IMBH do not
exhibit a cusp in the centre.
Belokurov V, Koposov SE, Evans NW, Peñarrubia J, Irwin MJ, Smith MC, Lewis GF, Gieles M, Wilkinson MI, Gilmore G, Olszewski EW, Niederste-Ostholt MN (2013) Precession of the Sagittarius stream,
Using a variety of stellar tracers -- blue horizontal branch stars,
main-sequence turn-off stars and red giants -- we follow the path of the
Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our
study presents new Sgr debris detections, accurate distances and line-of-sight
velocities that together help to shed new light on the puzzle of the Sgr tails.
For both the leading and the trailing tail, we trace the points of their
maximal extent, or apo-centric distances, and find that they lie at $R^L$ =
47.8 $\pm$ 0.5 kpc and $R^T$ = 102.5 $\pm$ 2.5 kpc respectively. The angular
difference between the apo-centres is 93.2 $\pm$ 3.5 deg, which is smaller than
predicted for logarithmic haloes. Such differential orbital precession can be
made consistent with models of the Milky Way in which the dark matter density
falls more quickly with radius. However, currently, no existing Sgr disruption
simulation can explain the entirety of the observational data. Based on its
position and radial velocity, we show that the unusually large globular cluster
NGC 2419 can be associated with the Sgr trailing stream. We measure the
precession of the orbital plane of the Sgr debris in the Milky Way potential
and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves
differently to the secondary (fainter) tails, both in the North and the South.
Bastian N, Weisz DR, Skillman ED, McQuinn KBW, Dolphin AE, Gutermuth RA, Cannon JM, Ercolano B, Gieles M, Kennicutt RC, Walter F (2010) The evolution of stellar structures in dwarf galaxies, Monthly Notices of the Royal Astronomical Society
We present a study of the variation of spatial structure of stellar
populations within dwarf galaxies as a function of the population age. We use
deep Hubble Space Telescope/Advanced Camera for Surveys imaging of nearby dwarf
galaxies in order to resolve individual stars and create composite
colour-magnitude diagrams (CMDs) for each galaxy. Using the obtained CMDs, we
select Blue Helium Burning stars (BHeBs), which can be unambiguously age-dated
by comparing the absolute magnitude of individual stars with stellar
isochrones. Additionally, we select a very young ( stars for a subset of the galaxies based on the tip of the young main-sequence.
By selecting stars in different age ranges we can then study how the spatial
distribution of these stars evolves with time. We find, in agreement with
previous studies, that stars are born within galaxies with a high degree of
substructure which is made up of a continuous distribution of clusters, groups
and associations from parsec to hundreds of parsec scales. These structures
disperse on timescales of tens to hundreds of Myr, which we quantify using the
two-point correlation function and the Q-parameter developed by Cartwright &
Whitworth (2004). On galactic scales, we can place lower limits on the time it
takes to remove the original structure (i.e., structure survives for at least
this long), tevo, which varies between ~100~Myr (NGC~2366) and ~350 Myr
(DDO~165). This is similar to what we have found previously for the SMC
(~80~Myr) and the LMC (~175 Myr). We do not find any strong correlations
between tevo and the luminosity of the host galaxy.
Bressert E, Bastian N, Evans CJ, Sana H, Hénault-Brunet V, Goodwin SP, Parker RJ, Gieles M, Bestenlehner JM, Vink JS, Taylor WD, Crowther PA, Longmore SN, Gräfener G, Apellániz JM, Koter AD, Cantiello M, Kruijssen JMD (2012) The VLT-FLAMES Tarantula Survey IV: Candidates for isolated high-mass
star formation in 30 Doradus,
Astronomy and Astrophysics
Whether massive stars can occasionally form in relative isolation or if they
require a large cluster of lower-mass stars around them is a key test in the
differentiation of star formation theories as well as how the initial mass
function of stars is sampled. Previous attempts to find O-type stars that
formed in isolation were hindered by the possibility that such stars are merely
runaways from clusters, i.e., their current isolation does not reflect their
birth conditions. We introduce a new method to find O-type stars that are not
affected by such a degeneracy. Using the VLT-FLAMES Tarantula Survey and
additional high resolution imaging we have identified stars that satisfy the
following constraints: 1) they are O-type stars that are not detected to be
part of a binary system based on RV time series analysis; 2) they are
designated spectral type O7 or earlier ; 3) their velocities are within 1\sigma
of the mean of OB-type stars in the 30 Doradus region, i.e. they are not
runaways along our line-of-sight; 4) the projected surface density of stars
does not increase within 3 pc towards the O-star (no evidence for clusters); 5)
their sight lines are associated with gaseous and/or dusty filaments in the
ISM, and 6) if a second candidate is found in the direction of the same
filament with which the target is associated, both are required to have similar
velocities. With these criteria, we have identified 15 stars in the 30 Doradus
region, which are strong candidates for being high-mass stars that have formed
in isolation. Additionally, we employed extensive MC stellar cluster
simulations to confirm that our results rule out the presence of clusters
around the candidates. Eleven of these are classified as Vz stars, possibly
associated with the zero-age main sequence. We include a newly discovered W-R
star as a candidate, although it does not meet all of the above criteria.
Anders P, Gieles M, Grijs RD (2006) Accurate photometry of extended spherically symmetric sources, A 451
We present a new method to derive reliable photometry of extended spherically
symmetric sources from {\it HST} images (WFPC2, ACS/WFC and NICMOS/NIC2
cameras), extending existing studies of point sources and marginally resolved
sources. We develop a new approach to accurately determine intrinsic sizes of
extended spherically symmetric sources, such as star clusters in galaxies
beyond the Local Group (at distances cookbook to perform aperture photometry on such sources, by determining
size-dependent aperture corrections (ACs) and taking sky oversubtraction as a
function of source size into account. In an extensive Appendix, we provide the
parameters of polynomial relations between the FWHM of various input profiles
and those obtained by fitting a Gaussian profile (which we have used for
reasons of computational robustness, although the exact model profile used is
irrelevant), and between the intrinsic and measured FWHM of the cluster and the
derived AC. Both relations are given for a number of physically relevant
cluster light profiles, intrinsic and observational parameters. AC relations
are provided for a wide range of apertures. Depending on the size of the source
and the annuli used for the photometry, the absolute magnitude of such extended
objects can be underestimated by up to 3 mag, corresponding to an error in mass
of a factor of 15. We carefully compare our results to those from the more
widely used DeltaMag method, and find an improvement of a factor of 3--40 in
both the size determination and the AC.
Hollyhead K, Adamo A, Bastian N, Gieles M, Ryon JE (2016) Properties of the cluster population of NGC 1566 and their implications, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 460 (2) pp. 2087-2102 OXFORD UNIV PRESS
Bastian N, Adamo A, Gieles M, Villa ES, Lamers HJGLM, Larsen SS, Smith LJ, Konstantopoulos IS, Zackrisson E (2011) Stellar Clusters in M83: Formation, evolution, disruption and the
influence of environment,
Monthly Notices of the Royal Astronomical Society
We study the stellar cluster population in two adjacent fields in the nearby,
face-on spiral galaxy, M83, using WFC3/HST imaging. The clusters are selected
through visual inspection to be centrally concentrated, symmetric, and resolved
on the images, which allows us to differentiate between clusters and likely
unbound associations. We compare our sample with previous studies and show that
the differences between the catalogues are largely due to the inclusion of
large numbers of diffuse associations within previous catalogues. The
luminosity function of the clusters is well approximated by a power-law with
index, -2, over most of the observed range, however a steepening is seen at M_V
= -9.3 and -8.8 in the inner and outer fields, respectively. Additionally, we
show that the cluster population is inconsistent with a pure power-law mass
distribution, but instead exhibits a truncation at the high mass end. If
described as a Schechter function, the characteristic mass is 1.6 and 0.5 *
10^5 Msun, for the inner and outer fields, respectively, in agreement with
previous estimates of other cluster populations in spiral galaxies. Comparing
the predictions of the mass independent disruption (MID) and mass dependent
disruption (MDD) scenarios with the observed distributions, we find that both
models can accurately fit the data. However, for the MID case, the fraction of
clusters destroyed (or mass lost) per decade in age is dependent on the
environment, hence, the age/mass distributions of clusters are not universal.
In the MDD case, the disruption timescale scales with galactocentric distance
(being longer in the outer regions of the galaxy) in agreement with analytic
and numerical predictions. Finally, we discuss the implications of our results
on other extragalactic surveys, focussing on the fraction of stars that form in
clusters and the need (or lack thereof) for infant mortality.
Cai MX, Gieles M, Heggie DC, Varri AL (2015) Evolution of star clusters on eccentric orbits, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 455 (1) pp. 596-602 OXFORD UNIV PRESS
Bastian N, Lamers HJGLM, Mink SED, Longmore SN, Goodwin SP, Gieles M (2013) Early Disc Accretion as the Origin of Abundance Anomalies in Globular
Clusters,
Globular clusters (GCs), once thought to be well approximated as simple
stellar populations (i.e. all stars having the same age and chemical
abundance), are now known to host a variety of anomalies, such as multiple
discrete (or spreads in) populations in colour-magnitude diagrams and abundance
variations in light elements (e.g., Na, O, Al). Multiple models have been put
forward to explain the observed anomalies, although all have serious
shortcomings (e.g., requiring a non-standard initial mass function of stars and
GCs to have been initially 10-100 times more massive than observed today).
These models also do not agree with observations of massive stellar clusters
forming today, which do not display significant age spreads nor have gas/dust
within the cluster. Here we present a model for the formation of GCs, where low
mass pre-main sequence (PMS) stars accrete enriched material released from
interacting massive binary and rapidly rotating stars onto their circumstellar
discs, and ultimately onto the young stars. As was shown in previous studies,
the accreted material matches the unusual abundances and patterns observed in
GCs. The proposed model does not require multiple generations of
star-formation, conforms to known properties of massive clusters forming today,
and solves the "mass budget problem" without requiring GCs to have been
significantly more massive at birth. Potential caveats to the model as well as
model predictions are discussed.
Bastian N, Ercolano B, Gieles M (2009) Hierarchical star formation in M33: Properties of the star-forming regions, Astrophysics and Space Science 324 (2) pp. 293-297
Star formation within galaxies occurs on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often as substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the Local Group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies, which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (~20 pc) to kpc scales. Once the groups have been identified, we study their properties, such as their size and luminosity distributions, and compare these with studies of young star clusters and giant molecular clouds (GMCs). We find evidence for a continuum of star-forming group sizes, which extends into the star cluster spatial-scale regime. We do not find a characteristic scale for OB associations, unlike that found in previous studies, and we suggest that the appearance of such a scale was caused by spatial resolution and selection effects. The luminosity function of the groups is found to be well represented by a power law with an index of -2, as has also been found for the luminosity and mass functions of young star clusters, as well as for the mass function of GMCs. Additionally, the groups follow a similar mass-radius relation as GMCs. The size distribution of the groups is best described by a lognormal distribution, the peak of which is controlled by the spatial scale probed and the minimum number of sources used to define a group. We show that within a hierarchical distribution, if a scale is selected to find structure, the resulting size distribution will have a log-normal distribution. We find an abrupt drop of the number of groups outside a galactic radius of ~4 kpc (although individual high-mass stars are found beyond this limit), suggesting a change in the structure of the star-forming interstellar medium, possibly reflected in the lack of GMCs beyond this radius. Finally, we find that the spatial distribution of H II regions, GMCs, and star-forming groups are all highly correlated. © Springer Science+Business Media B.V. 2009.
Gieles M (2012) Mass loss of stars in star clusters: an energy source for dynamical
evolution,
Dense star clusters expand until their sizes are limited by the tidal field
of their host galaxy. During this expansion phase the member stars evolve and
lose mass. We show that for clusters with short initial relaxation time scales
( in the core, but happens on a relaxation time scale. That is, the energy
release following stellar mass loss is in balance with the amount of energy
that is transported outward by two-body relaxation.
Campbell MA, Evans CJ, Mackey AD, Gieles M, Alves J, Ascenso J, Bastian N, Longmore AJ (2010) VLT-MAD observations of the core of 30 Doradus, Monthly Notices of the Royal Astronomical Society
We present H- and Ks-band imaging of three fields at the centre of 30 Doradus
in the Large Magellanic Cloud, obtained as part of the Science Demonstration
programme with the Multi-conjugate Adaptive optics Demonstrator (MAD) at the
Very Large Telescope. Strehl ratios of 15-30% were achieved in the Ks-band,
yielding near-infrared images of this dense and complex region at unprecedented
angular resolution at these wavelengths. The MAD data are used to construct a
near-infrared luminosity profile for R136, the cluster at the core of 30 Dor.
Using cluster profiles of the form used by Elson et al., we find the surface
brightness can be fit by a relatively shallow power-law function
(gamma~1.5-1.7) over the full extent of the MAD data, which extends to a radius
of ~40" (~10pc). We do not see compelling evidence for a break in the
luminosity profile as seen in optical data in the literature, arguing that
cluster asymmetries are the dominant source, although extinction effects and
stars from nearby triggered star-formation likely also contribute. These
results highlight the need to consider cluster asymmetries and multiple spatial
components in interpretation of the luminosity profiles of distant unresolved
clusters. We also investigate seven candidate young stellar objects reported by
Gruendl & Chu from Spitzer observations, six of which have apparent
counterparts in the MAD images. The most interesting of these (GC09:
053839.24-690552.3) appears related to a striking bow-shock--like feature,
orientated away from both R136 and the Wolf-Rayet star Brey 75, at distances of
19.5" and 8" (4.7 and 1.9pc in projection), respectively.
Smith LJ, Bastian N, Konstantopoulos IS, Gallagher JS, Gieles M, Grijs RD, Larsen SS, O'Connell RW, Westmoquette MS (2007) The Young Cluster Population of M82 Region B,
We present observations obtained with the Advanced Camera for Surveys on
board the Hubble Space Telescope of the "fossil" starburst region B in the
nearby starburst galaxy M82. By comparing UBVI photometry with models, we
derive ages and extinctions for 35 U-band selected star clusters. We find that
the peak epoch of cluster formation occurred ~ 150 Myr ago, in contrast to
earlier work that found a peak formation age of 1.1 Gyr. The difference is most
likely due to our inclusion of U-band data, which are essential for accurate
age determinations of young cluster populations. We further show that the
previously reported turnover in the cluster luminosity function is probably due
to the neglect of the effect of extended sources on the detection limit. The
much younger cluster ages we derive clarifies the evolution of the M82
starburst. The M82-B age distribution now overlaps with the ages of: the
nuclear starburst; clusters formed on the opposite side of the disk; and the
last encounter with M81, some 220 Myr ago.
Gieles M (2013) The mass and radius evolution of globular clusters in tidal fields,
We present a simple theory for the evolution of initially compact clusters in
a tidal field. The fundamental ingredient of the model is that a cluster
conducts a constant fraction of its own energy through the half-mass radius by
two-body interactions every half-mass relaxation time. This energy is produced
in a self-regulative way in the core by an (unspecified) energy source. We find
that the half-mass radius increases during the first part (roughly half) of the
evolution and decreases in the second half, while the escape rate is constant
and set by the tidal field. We present evolutionary tracks and isochrones for
clusters in terms of cluster half-mass density, cluster mass and
galacto-centric radius. We find substantial agreement between model isochrones
and Milky Way globular cluster parameters, which suggests that there is a
balance between the flow of energy and the central energy production for almost
all globular clusters. We also find that the majority of the globular clusters
are still expanding towards their tidal radius. Finally, a fast code for
cluster evolution is presented.
Gieles M, Larsen S, Bastian N, Stein I (2005) The luminosity function of young star clusters: implications for the
maximum mass and luminosity of clusters,
We introduce a method to relate a possible truncation of the star cluster
mass function at the high mass end to the shape of the cluster luminosity
function (LF). We compare the observed LFs of five galaxies containing young
star clusters with synthetic cluster population models with varying initial
conditions. The LF of the SMC, the LMC and NGC 5236 are characterized by a
power-law behavior NdL~L^-a dL, with a mean exponent of = 2.0 +/- 0.2. This
can be explained by a cluster population formed with a constant cluster
formation rate, in which the maximum cluster mass per logarithmic age bin is
determined by the size-of-sample effect and therefore increases with
log(age/yr). The LFs of NGC 6946 and M51 are better described by a double
power-law distribution or a Schechter function. When a cluster population has a
mass function that is truncated below the limit given by the size-of-sample
effect, the total LF shows a bend at the magnitude of the maximum mass, with
the age of the oldest cluster in the population, typically a few Gyr due to
disruption. For NGC 6946 and M51 this implies a maximum mass of M_max = 5*10^5
M_sun. Faint-ward of the bend the LF has the same slope as the underlying
initial cluster mass function and bright-ward of the bend it is steeper. This
behavior can be well explained by our population model. We compare our results
with the only other galaxy for which a bend in the LF has been observed, the
``Antennae'' galaxies (NGC 4038/4039). There the bend occurs brighter than in
NGC 6946 and M51, corresponding to a maximum cluster mass of M_max = 2*10^6
M_sun (abridged).
Zwart SP, McMillan S, Gieles M (2010) Young massive star clusters, Annual Review of Astronomy and Astrophysics
Young massive clusters are dense aggregates of young stars that form the
fundamental building blocks of galaxies. Several examples exist in the Milky
Way Galaxy and the Local Group, but they are particularly abundant in starburst
and interacting galaxies. The few young massive clusters that are close enough
to resolve are of prime interest for studying the stellar mass function and the
ecological interplay between stellar evolution and stellar dynamics. The
distant unresolved clusters may be effectively used to study the star-cluster
mass function, and they provide excellent constraints on the formation
mechanisms of young cluster populations. Young massive clusters are expected to
be the nurseries for many unusual objects, including a wide range of exotic
stars and binaries. So far only a few such objects have been found in young
massive clusters, although their older cousins, the globular clusters, are
unusually rich in stellar exotica. In this review we focus on star clusters
younger than $\sim100$ Myr, more than a few current crossing times old, and
more massive than $\sim10^4$ \Msun, irrespective of cluster size or
environment. We describe the global properties of the currently known young
massive star clusters in the Local Group and beyond, and discuss the state of
the art in observations and dynamical modeling of these systems. In order to
make this review readable by observers, theorists, and computational
astrophysicists, we also review the cross-disciplinary terminology.
Gieles M, Zwart SFP, Athanassoula E (2006) The effect of giant molecular clouds on star clusters,
We study the encounters between stars clusters and giant molecular clouds
(GMCs). The effect of these encounters has previously been studied analytically
for two cases: 1) head-on encounters, for which the cluster moves through the
centre of the GMC and 2) distant encounters, where the encounter distance p >
3*R_n, with p the encounter parameter and R_n the radius of the GMC. We
introduce an expression for the energy gain of the cluster due to GMC
encounters valid for all values of p and R_n. This analytical result is
confronted with results from N-body simulations and excellent agreement is
found. From the simulations we find that the fractional mass loss is only 25%
of the fractional energy gain. This is because stars escape with velocities
much higher than the escape velocity. Based on the mass loss, we derive a
disruption time for star clusters due to encounters with GMCs of the form t_dis
[Gyr] = 2.0*S*(M_c/10^4 M_sun)^gamma, with S=1 for the solar neighbourhood and
inversely proportional with the global GMC density and gamma=1-3lambda, with
lambda the index that relates the cluster half-mass radius to the cluster mass
(r_h ~ M_c^lambda). The observed shallow relation between cluster radius and
mass (e.g. lambda=0.1), makes the index (gamma=0.7) similar to the index found
both from observations and from simulations of clusters dissolving in tidal
fields (gamma=0.62). The constant of 2.0 Gyr, which is the disruption time of a
10^4 M_sun cluster in the solar neighbourhood, is close to the value of 1.3 Gyr
which was empirically determined from the age distribution of open clusters.
This suggests that the combined effect of GMC encounters, stellar evolution and
galactic tidal field can explain the lack of old open clusters in the solar
neighbourhood.
Gieles M, Larsen SS, Haas MR, Scheepmaker RA, Bastian N (2006) The Maximum Mass of Star Clusters,
When an universal untruncated star cluster initial mass function (CIMF)
described by a power-law distribution is assumed, the mass of the most massive
star cluster in a galaxy (M_max) is the result of the size-of-sample (SoS)
effect. This implies a dependence of M_max on the total number of star clusters
(N). The SoS effect also implies that M_max within a cluster population
increases with equal logarithmic intervals of age. This is because the number
of clusters formed in logarithmic age intervals increases (assuming a constant
cluster formation rate). This effect has been observed in the SMC and LMC.
Based on the maximum pressure (P_int) inside molecular clouds, it has been
suggested that a physical maximum mass (M_max[phys]) should exist. The theory
predicts that M_max[phys] should be observable, i.e. lower than M_max that
follows from statistical arguments, in big galaxies with a high star formation
rate. We compare the SoS relations in the SMC and LMC with the ones in M51 and
model the integrated cluster luminosity function (CLF) for two cases: 1) M_max
is determined by the SoS effect and 2) M_max=M_max[phys]=constant. The observed
CLF of M51 and the comparison of the SoS relations with the SMC and LMC both
suggest that there exists a M_max[phys] of 5*10^5 M_sun in M51. The CLF of M51
looks very similar to the one observed in the ``Antennae'' galaxies. A direct
comparison with our model suggests that there M_max[phys]=2*10^6 M_sun.
Gieles M (2011) Dynamical evolution of stellar clusters,
The evolution of star clusters is determined by several internal and external
processes. Here we focus on two dominant internal effects, namely energy
exchange between stars through close encounters (two-body relaxation) and
mass-loss of the member stars through stellar winds and supernovae explosions.
Despite the fact that the former operates on the relaxation timescale of the
cluster and the latter on a stellar evolution timescale, these processes work
together in driving a nearly self-similar expansion, without forming (hard)
binaries. Low-mass clusters expand more, such that after some time the radii of
clusters depend very little on their masses, even if all clusters have the same
(surface) density initially. Throughout it is assumed that star clusters are in
virial equilibrium and well within their tidal boundary shortly after
formation, motivated by observations of young (few Myrs) clusters. We start
with a discussion on how star clusters can be distinguished from (unbound)
associations at these young ages.
Gieles M (2009) Basic Tools for Studies on the Formation and Disruption of Star
Clusters: the Luminosity Function,
Arxiv
The luminosity function (LF) of young star clusters is often approximated by
a power law function. For clusters in a wide range of galactic environments
this has resulted in fit indices near -2, but on average slightly steeper. A
fundamental property of the -2 power law function is that the luminosity of the
brightest object (L_max) scales linearly with the total number of clusters,
which is close to what is observed. This suggests that the formation of Young
Massive Clusters (YMCs) is a result of the size of the sample, i.e. when the
SFR is high it is statistically more likely to form YMCs, but no particular
physical conditions are required. In this contribution we provide evidence that
the LF of young clusters is not a -2 power law, but instead is curved, showing
a systematic decrease of the (logarithmic) slope from roughly -1.8 at low
luminosities to roughly -2.8 at high luminosities. The empirical LFs can be
reproduced by model LFs using an underlying cluster IMF with a Schechter type
truncation around M*=2x10^5 M_sun. This value of M* can not be universal since
YMCs well in excess of this M* are known in merging galaxies and merger
remnants. Therefore, forming super massive clusters (>10^6 M_sun) probably
requires conditions different from those in (quiescent) spiral galaxies and
hence is not only the result of a size-of-sample effect. From the vertical
offset a cluster formation efficiency of ~10% is derived. We find indications
for this efficiency to be higher when the SFR is higher.
Sana H, Mink SED, Koter AD, Langer N, Evans CJ, Gieles M, Gosset E, Izzard RG, Bouquin J-BL, Schneider FRN (2012) Multiplicity of massive O stars and evolutionary implications,
Nearby companions alter the evolution of massive stars in binary systems.
Using a sample of Galactic massive stars in nearby young clusters, we
simultaneously measure all intrinsic binary characteristics relevant to
quantify the frequency and nature of binary interactions. We find a large
intrinsic binary fraction, a strong preference for short orbital periods and a
flat distribution for the mass-ratios. Our results do not support the presence
of a significant peak of equal-mass `twin' binaries. As a result of the
measured distributions, we find that over seventy per cent of all massive stars
exchange mass with a companion. Such a rate greatly exceeds previous estimates
and implies that the majority of massive stars have their evolution strongly
affected by interaction with a nearby companion.
Lamers HJGLM, Baumgardt H, Gieles M (2013) The evolution of the global stellar mass function of star clusters: an analytic description, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 433 (2) pp. 1378-1388 OXFORD UNIV PRESS
Gaburov E, Gieles M (2007) Integrated properties of mass segregated star clusters,
In this contribution we study integrated properties of dynamically segregated
star clusters. The observed core radii of segregated clusters can be 50%
smaller than the ``true'' core radius. In addition, the measured radius in the
red filters is smaller than those measured in blue filters. However, these
difference are small ($\lesssim10%$), making it observationally challenging to
detect mass segregation in extra-galactic clusters based on such a comparison.
Our results follow naturally from the fact that in nearly all filters most of
the light comes from the most massive stars. Therefore, the observed surface
brightness profile is dominated by stars of similar mass, which are centrally
concentrated and have a similar spatial distribution.
Gieles M, Alexander P, Lamers H, Baumgardt H (2013) A prescription and fast code for the long-term evolution of star
clusters - II. Unbalanced and core evolution,
Mon. Not. R. Astron. Soc. 437 pp. 916-929
We introduce version two of the fast star cluster evolution code Evolve Me A
Cluster of StarS (EMACSS). The first version (Alexander & Gieles) assumed that
cluster evolution is balanced for the majority of the life-cycle, meaning that
the rate of energy generation in the core of the cluster equals the diffusion
rate of energy by two-body relaxation, which makes the code suitable for
modelling clusters in weak tidal fields. In this new version we extend the
model to include an unbalanced phase of evolution to describe the pre-collapse
evolution and the accompanying escape rate such that clusters in strong tidal
fields can also be modelled. We also add a prescription for the evolution of
the core radius and density and a related cluster concentration parameter. The
model simultaneously solves a series of first-order ordinary differential
equations for the rate of change of the core radius, half-mass radius and the
number of member stars N. About two thousand integration steps in time are
required to solve for the entire evolution of a star cluster and this number is
approximately independent of N. We compare the model to the variation of these
parameters following from a series of direct N-body calculations of single-mass
clusters and find good agreement in the evolution of all parameters. Relevant
time-scales, such as the total lifetimes and core collapse times, are
reproduced with an accuracy of about 10% for clusters with various initial
half-mass radii (relative to their Jacobi radii) and a range of different
initial N up to N = 65536. We intend to extend this framework to include more
realistic initial conditions, such as a stellar mass spectrum and mass loss
from stars. The EMACSS code can be used in star cluster population studies and
in models that consider the co-evolution of (globular) star clusters and large
scale structures.
Scheepmaker RA, Gieles M, Haas MR, Bastian N, Larsen SS, Lamers HJGLM (2006) The radii of thousands of star clusters in M51 with HST/ACS,
We exploit the superb resolution of the new HST/ACS mosaic image of M51 to
select a large sample of young ( based on their sizes. The image covers the entire spiral disk in B, V, I and
H_alpha, at a resolution of 2 pc per pixel. The surface density distribution of
4357 resolved clusters shows that the clusters are more correlated with clouds
than with stars, and we find a hint of enhanced cluster formation at the
corotation radius. The radius distribution of a sample of 769 clusters with
more accurate radii suggests that young star clusters have a preferred
effective radius of ~3 pc, which is similar to the preferred radius of the much
older GCs. However, in contrast to the GCs, the young clusters in M51 do not
show a relation between radius and galactocentric distance. This means that the
clusters did not form in tidal equilibrium with their host galaxy, nor that
their radius is related to the ambient pressure.
Lamers HJGLM, Gieles M (2006) Clusters in the solar neighbourhood: how are they destroyed?,
We predict the survival time of initially bound star clusters in the solar
neighbourhood taking into account: (1) stellar evolution, (2) tidal stripping,
(3) shocking by spiral arms and (4) encounters with giant molecular clouds. We
find that the predicted dissolution time is t_dis= 1.7 (M_i/10^4 M_sun)^0.67
Gyr for clusters in the mass range of 10^2 M_i is the initial mass of the cluster.. The resulting predicted shape of the
logarithmic age distribution agrees very well with the empirical one, derived
from a complete sample of clusters in the solar neighbourhood within 600 pc.
The required scaling factor implies a star formation rate of 400 M_sun/Myr
within 600 pc from the Sun or a surface formation rate of 3.5 10^-10 M_sun/(yr
pc^2) for stars in bound clusters with an initial mass in the range of 10^2 to
3 10^4 M_sun.
Gieles M, Baumgardt H, Bastian N, Lamers HJGLM (2004) Theoretical and Observational Agreement on Mass Dependence of Cluster
Life Times,
Observations and N-body simulations both support a simple relation for the
disruption time of a cluster as a function of its mass of the form: t_dis = t_4
* (M/10^4 Msun)^gamma. The scaling factor t_4 seems to depend strongly on the
environment. Predictions and observations show that gamma ~ 0.64 +/- 0.06.
Assuming that t_dis ~ M^0.64 is caused by evaporation and shocking implies a
relation between the radius and the mass of a cluster of the form: r_h ~
M^0.07, which has been observed in a few galaxies. The suggested relation for
the disruption time implies that the lower mass end of the cluster initial mass
function will be disrupted faster than the higher mass end, which is needed to
evolve a young power law shaped mass function into the log-normal mass function
of old (globular) clusters.
Gieles M (2009) What determines the mass of the most massive star cluster in a galaxy: Statistics, physics or disruption?, Astrophysics and Space Science 324 (2) pp. 299-304
In many different galactic environments the cluster initialmass function (CIMF) is well described by a power law with index-2. This implies a linear relation between the mass of the most massive cluster (Mmax) and the number of clusters. Assuming a constant cluster formation rate and no disruption of the most massive clusters it also means that Mmax increases linearly with age when determining Mmax in logarithmic age bins. We observe this increase in five out of the seven galaxies in our sample, suggesting that Mmax is determined by the size of the sample. It also means that massive clusters are very stable against disruption, in disagreement with the mass-independent disruption (MID) model. For the clusters in M51 and the Antennae galaxies, the sizeof- sample prediction breaks down around 106 M, suggesting that this is a physical upper limit to the masses of star clusters in these galaxies. In this method there is a degeneracy between MID and a CIMF truncation.We show how the cluster luminosity function can serve as a tool to distinguish between the two. © Springer Science+Business Media B.V. 2009.
Renaud F, Gieles M (2015) The effect of secular galactic growth on the evolution of star clusters,
The growth of galaxies through adiabatic accretion of dark matter is one of
the main drivers of galaxy evolution. By isolating it from other processes like
mergers, we analyse how it affects the evolution of star clusters. Our study
comprises a fast and approximate exploration of the orbital and intrinsic
cluster parameter space, and more detailed monitoring of their evolution,
through N-body simulations for a handful of cases. We find that the properties
of present-day star clusters and their tidal tails differ very little, whether
the clusters are embedded in a growing galactic halo for 12 Gyr, or in a static
one.
Lamers HJGLM, Gieles M, Zwart SFP (2004) Disruption time scales of star clusters in different galaxies, Astron.Astrophys. 429 pp. 173-179
The observed average lifetime of the population of star clusters in the Solar
Neighbourhood, the Small Magellanic Cloud and in selected regions of M51 and
M33 is compared with simple theoretical predictions and with the results of
N-body simulations. The empirically derived lifetimes (or disruption times) of
star clusters depend on their initial mass as t_dis ~ Mcl^0.60 in all four
galaxies. N-body simulations have shown that the predicted disruption time of
clusters in a tidal field scales as t_dis^pred ~ t_rh^0.75 t_cr^0.25, where
t_rh is the initial half-mass relaxation time and t_cr is the crossing time for
a cluster in equilibrium. We show that this can be approximated accurately by
t_dis^pred ~ M_cl^0.62 for clusters in the mass range of about 10^3 to 10^6
M_sun, in excellent agreement with the observations. Observations of clusters
in different extragalactic environments show that t_dis also depends on the
ambient density in the galaxies where the clusters reside. Linear analysis
predicts that the disruption time will depend on the ambient density of the
cluster environment as t_dis ~ rho_amb^-0.5. This relation is consistent with
N-body simulations.
Carballo-Bello JA, Gieles M, Sollima A, Koposov S, Martínez-Delgado D, Peñarrubia J (2011) Outer density profiles of 19 Galactic globular clusters from deep and
wide-field imaging,
Monthly Notices of the Royal Astronomical Society
Using deep photometric data from WFC@INT and WFI@ESO2.2m we measure the outer
number density profiles of 19 stellar clusters located in the inner region of
the Milky Way halo (within a Galactocentric distance range of 10-30 kpc) in
order to assess the impact of internal and external dynamical processes on the
spatial distribution of stars. Adopting power-law fitting templates, with index
$-\gamma$ in the outer region, we find that the clusters in our sample can be
divided in two groups: a group of massive clusters ($ \ge 10^5 $ M_sun) that
has relatively flat profiles with $2.5 clusters ($ \le 10^5 $ M_sun), with steep profiles ($\gamma > 4$) and clear
signatures of interaction with the Galactic tidal field. We refer to these two
groups as 'tidally unaffected' and 'tidally affected', respectively. Our
results also show a clear trend between the slope of the outer parts and the
half-mass density of these systems, which suggests that the outer density
profiles may retain key information on the dominant processes driving the
dynamical evolution of Globular Clusters.
Gieles M, Larsen S, Scheepmaker R, Bastian N, Haas M, Lamers H (2005) Observational evidence for a truncation of the star cluster initial mass
function at the high mass end,
We present the luminosity function (LF) of star clusters in M51 based on
HST/ACS observations taken as part of the Hubble Heritage project. The clusters
are selected based on their size and with the resulting 5990 clusters we
present one of the largest cluster samples of a single galaxy. We find that the
LF can be approximated with a double power-law distribution with a break around
M_V = -8.9. On the bright side the index of the power-law distribution is
steeper (a = 2.75) than on the faint-side (a = 1.93), similar to what was found
earlier for the ``Antennae'' galaxies. The location of the bend, however,
occurs about 1.6 mag fainter in M51. We confront the observed LF with the model
for the evolution of integrated properties of cluster populations of Gieles et
al., which predicts that a truncated cluster initial mass function would result
in a bend in, and a double power-law behaviour of, the integrated LF. The
combination of the large field-of view and the high star cluster formation rate
of M51 make it possible to detect such a bend in the LF. Hence, we conclude
that there exists a fundamental upper limit to the mass of star clusters in
M51. Assuming a power-law cluster initial mass function with exponentional
cut-off of the form NdM ~ M^-b * exp(-M/M_C)dM, we find that M_C = 10^5 M_sun.
A direct comparison with the LF of the ``Antennae'' suggests that there M_C =
4*10^5 M_sun.
Gieles M (2006) The role of tidal forces in star cluster disruption,
Star clusters are subject to density irregularities in their host galaxy,
such as giant molecular clouds (GMCs), the galactic disc and spiral arms, which
are largely ignored in present day (N-body) simulations of cluster evolution.
Time dependent external potentials give rise to tidal forces that accelerate
stars leading to an expansion and more rapid dissolution of the cluster. I
explain the basic principles of this tidal heating in the impulse approximation
and show how related disruption time-scales depend on properties of the
cluster.
Bastian N, Ercolano B, Gieles M (2009) Hierarchical star formation in M33: properties of the star-forming regions, Astrophysics and Space Science pp. 1-5
Star formation within galaxies occurs on multiple scales, from spiral structure, to OB associations, to individual star clusters, and often as substructure within these clusters. This multitude of scales calls for objective methods to find and classify star-forming regions, regardless of spatial size. To this end, we present an analysis of star-forming groups in the Local Group spiral galaxy M33, based on a new implementation of the Minimum Spanning Tree (MST) method. Unlike previous studies, which limited themselves to a single spatial scale, we study star-forming structures from the effective resolution limit (
Contenta F, Gieles M, Balbinot E, Collins MLM (2016) The contribution of dissolving star clusters to the population of ultra faint objects in the outer halo of the Milky Way, Monthly Notices of the Royal Astronomical Society 466 (2) pp. 1741-1756 Oxford Univercity Press
In the last decade, several ultra faint objects (UFOs, MV ?3.5) have been discovered in the outer halo of the Milky Way. For some of these objects, it is not clear whether they are star clusters or (ultra faint) dwarf galaxies. In this work, we quantify the contribution of star clusters to the population of UFOs. We extrapolated the mass and Galactocentric radius distribution of the globular clusters using a population model, finding that the Milky Way contains about 3.3+7.3 ?1.6 star clusters with MV ?3.5 and Galactocentric radius e20 kpc. To understand whether dissolving clusters can appear as UFOs, we run a suite of direct N-body models, varying the orbit, the Galactic potential, the binary fraction and the black hole (BH) natal kick velocities. In the analyses, we consider observational biases such as luminosity limit, field stars and line-of-sight projection. We find that star clusters contribute to both the compact and the extended population of UFOs: clusters without BHs appear compact with radii
Renaud F, Agertz O, Gieles M (2017) The origin of the Milky Way globular clusters, Monthly Notices of the Royal Astronomical Society 465 (3) pp. 3622-3636 Oxford University Press
We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies which are accreted onto the Milky Way, while the red, metal-rich clusters form mostly in situ or, to a lower extent in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present-day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present-day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field.
Claydon I, Gieles M, Zocchi A (2017) The properties of energetically unbound stars in stellar clusters, Monthly Notices of the Royal Astronomical Society 466 (4) pp. 3937-3950 Oxford University Press
Several Milky Way star clusters show a roughly flat velocity dispersion profile at large radii, which is not expected from models with a tidal cut-off energy. Possible explanations for this excess velocity include: the effects of a dark matter halo, modified gravity theories and energetically unbound stars inside of clusters. These stars are known as potential escapers (PEs) and can exist indefinitely within clusters which are on circular orbits. Through a series of N-body simulations of star cluster systems, where we vary the galactic potential, orbital eccentricity and stellar mass function, we investigate the properties of the PEs and their effects on the kinematics. We derive a prediction for the scaling of the velocity dispersion at the Jacobi surface due to PEs, as a function of cluster mass, angular velocity of the cluster orbit, and slope of the mass profile of the host galaxy. We see a tentative signal of the mass and orbital velocity dependence in kinematic data of globular clusters from literature. We also find that the fraction of PEs depends sensitively on the galactic mass profile, reaching as high as 40% in the cusp of a Navarro-Frenk-White profile and as the velocity anisotropy also depends on the slope of the galactic mass profile, we conclude that PEs provide an independent way of inferring the properties of the dark matter mass profile at the galactic radius of (globular) clusters in the Gaia-era
Peuten M, Zocchi A, Gieles M, Gualandris A, Henault-Brunet V (2016) A stellar-mass black hole population in the globular cluster NGC 6101?, Monthly Notices of the Royal Astronomical Society 462 (3) pp. 2333-2342 Oxford University Press
Dalessandro et al. observed a similar distribution for blue straggler stars and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and interpreted this feature as an indication that this cluster is not mass-segregated. Using direct N-body simulations, we find that a significant amount of mass segregation is expected for a cluster with the mass, radius and age of NGC 6101. Therefore, the absence of mass segregation cannot be explained by the argument that the cluster is not yet dynamically evolved. By varying the retention fraction of stellar-mass black holes, we show that segregation is not observable in clusters with a high black hole retention fraction (>50 per cent after supernova kicks and >50 per cent after dynamical evolution). Yet all model clusters have the same amount of mass segregation in terms of the decline of the mean mass of stars and remnants with distance to the centre. We also discuss how kinematics can be used to further constrain the presence of a stellar-mass black hole population and distinguish it from the effect of an intermediate-mass black hole. Our results imply that the kick velocities of black holes are lower than those of neutron stars. The large retention fraction during its dynamical evolution can be explained if NGC 6101 formed with a large initial radius in a Milky Way satellite.
Gieles M, Renaud F (2016) If it does not kill them, it makes them stronger: collisional evolution of star clusters with tidal shocks, Monthly Notices of the Royal Astronomical Society 463 (1) pp. L103-L107 Oxford University Press
The radii of young (. 100 Myr) star clusters correlate only weakly with their masses. This shallow relation has been used to argue that impulsive tidal perturbations, or ?shocks?, by passing giant molecular clouds (GMCs) preferentially disrupt low-mass clusters. We show that this mass-radius relation is in fact the result of the combined effect of two-body relaxation and repeated tidal shocks. Clusters in a broad range of environments including those like the solar neighbourhood evolve towards a typical radius of a few parsecs, as observed, independent of the initial radius. This equilibrium mass-radius relation is the result of a competition between expansion by relaxation and shrinking due to shocks. Interactions with GMCs are more disruptive for low-mass clusters, which helps to evolve the globular cluster mass function (GCMF). However, the properties of the interstellar medium in high-redshift galaxies required to establish a universal GCMF shape are more extreme than previously derived, challenging the idea that all GCs formed with the same power-law mass function.
Peuten M, Zocchi A, Gieles M (2017) Testing lowered isothermal models with direct N-body
simulations of globular clusters - II. Multimass models,
Monthly Notices of the Royal Astronomical Society 470 (3) pp. 2736-2761 Oxford University Press
Lowered isothermal models, such as the multimass Michie?King models, have been successful in describing observational data of globular clusters. In this study, we assess whether such models are able to describe the phase space properties of evolutionary N-body models. We compare the multimass models as implemented in LIMEPY (Gieles & Zocchi) to N-body models of star clusters with different retention fractions for the black holes and neutron stars evolving in a tidal field. We find that multimass models successfully reproduce the density and velocity dispersion profiles of the different mass components in all evolutionary phases and for different remnants retention. We further use these results to study the evolution of global model parameters. We find that over the lifetime of clusters, radial anisotropy gradually evolves from the low- to the high-mass components and we identify features in the properties of observable stars that are indicative of the presence of stellar-mass black holes. We find that the model velocity scale depends on mass as m?´, with ´ C 0.5 for almost all models, but the dependence of central velocity dispersion on m can be shallower, depending on the dark remnant content, and agrees well with that of the N-body models. The reported model parameters, and correlations amongst them, can be used as theoretical priors when fitting these types of mass models to observational data.
Zocchi A, Gieles M, Henault-Brunet V (2017) Radial anisotropy in É Cen limiting the room for an intermediate-mass
black hole,
Monthly Notices of the Royal Astronomical Society 468 (4) pp. 4429-4440 Oxford University Press
Finding an intermediate-mass black hole (IMBH) in a globular cluster (or proving its absence) would provide valuable insights into our understanding of galaxy formation and evolution. However, it is challenging to identify a unique signature of an IMBH that cannot be accounted for by other processes. Observational claims of IMBH detection are indeed often based on analyses of the kinematics of stars in the cluster core, the most common signature being a rise in the velocity dispersion profile towards the centre of the system. Unfortunately, this IMBH signal is degenerate with the presence of radially-biased pressure anisotropy in the globular cluster. To explore the role of anisotropy in shaping the observational kinematics of clusters, we analyse the case of É Cen by comparing the observed profiles to those calculated from the family of LIMEPY models, that account for the presence of anisotropy in the system in a physicallymotivated way. The best-fit radially anisotropicmodels reproduce the observational profiles well, and describe the central kinematics as derived from Hubble Space Telescope proper motions without the need for an IMBH.
Almeida L, Sana H, Taylor W, Barbá R, Bonanos A, Crowther P, Damineli A, de Koter A, de Mink S, Evans C, Gieles M, Grin N, Hénault-Brunet V, Langer N, Lennon D, Lockwood S, Maíz Apellániz J, Moffat A, Neijssel C, Norman C, Ramírez-Agudelo O, Richardson N, Schootemeijer A, Shenar T, SoszyDski I, Tramper F, Vink J (2017) The Tarantula Massive Binary Monitoring. I. Observational campaign and OB-type spectroscopic binaries, Astronomy and Astrophysics 598 A84 EDP Sciences
ontext. Massive binaries play a crucial role in the Universe. Knowing the distributions of their orbital parameters is important for a wide range of topics from stellar feedback to binary evolution channels and from the distribution of supernova types to gravitational wave progenitors, yet no direct measurements exist outside the Milky Way. Aims. The Tarantula Massive Binary Monitoring project was designed to help fill this gap by obtaining multi-epoch radial velocity (RV) monitoring of 102 massive binaries in the 30 Doradus region. Methods. In this paper we analyze 32 FLAMES/GIRAFFE observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single-lined (SB1) and 31 double-lined (SB2) spectroscopic binaries. Results. Overall, the binary fraction and orbital properties across the 30 Doradus region are found to be similar to existing Galactic samples. This indicates that within these domains environmental effects are of second order in shaping the properties of massive binary systems. A small difference is found in the distribution of orbital periods, which is slightly flatter (in log space) in 30 Doradus than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, orbital periods in 30 Doradus can be as short as 1.1 d, somewhat shorter than seen in Galactic samples. Equal mass binaries (q> 0.95) in 30 Doradus are all found outside NGC 2070, the central association that surrounds R136a, the very young and massive cluster at 30 Doradus?s core. Most of the differences, albeit small, are compatible with expectations from binary evolution. One outstanding exception, however, is the fact that earlier spectral types (O2?O7) tend to have shorter orbital periods than later spectral types (O9.2?O9.7). Conclusions. Our results point to a relative universality of the incidence rate of massive binaries and their orbital properties in the metallicity range from solar (Z™) to about half solar. This provides the first direct constraints on massive binary properties in massive star-forming galaxies at the Universe?s peak of star formation at redshifts z ~ 1 to 2 which are estimated to have Z ~ 0.5 Z™.
Balbinot E, Gieles M (2017) The devil is in the tails: the role of globular cluster mass evolution on stream properties, Monthly Notices of the Royal Astronomical Society 474 (2) pp. 2479-2492 Oxford University Press (OUP)
We present a study of the effects of collisional dynamics on the formation and detectability
of cold tidal streams. A semi-analytical model for the evolution of the stellar mass function
was implemented and coupled to a fast stellar stream simulation code, as well as the synthetic
cluster evolution code EMACSS for the mass evolution as a function of a globular cluster
orbit. We find that the increase in the average mass of the escaping stars for clusters close
to dissolution has a major effect on the observable stream surface density. As an example,
we show that Palomar 5 would have undetectable streams (in an SDSS-like survey) if it was
currently three times more massive, despite the fact that a more massive cluster loses stars
at a higher rate. This bias due to the preferential escape of low-mass stars is an alternative
explanation for the absence of tails near massive clusters, than a dark matter halo associated
with the cluster. We explore the orbits of a large sample of Milky Way globular clusters and
derive their initial masses and remaining mass fraction. Using properties of known tidal tails
we explore regions of parameter space that favour the detectability of a stream. A list of high
probability candidates is discussed.
Gieles M, Balbinot E, Yaaqib R, Hénault-Brunet V, Zocchi A, Peuten M, Jonker P (2017) Mass models of NGC 6624 without an intermediate-mass black hole, Monthly Notices of the Royal Astronomical Society 473 (4) pp. 4832-4839 Oxford University Press (OUP)
An intermediate-mass black hole (IMBH) was recently reported to reside in the centre of the Galactic globular cluster (GC) NGC 6624, based on timing observations of a millisecond pulsar (MSP) located near the cluster centre in projection. We present dynamical models with multiple mass components of NGC 6624 ? without an IMBH ? which successfully describe the surface brightness profile and proper motion kinematics from the Hubble Space Telescope (HST) and the stellar mass function at different distances from the cluster centre. The maximum line-of-sight acceleration at the position of the MSP accommodates the inferred acceleration of the MSP, as derived from its first period derivative. With discrete realisations of the models we show that the higher-order period derivatives ? which were previously used to derive the IMBH mass ? are due to passing stars and stellar remnants, as previously shown analytically in literature. We conclude that there is no need for an IMBH to explain the timing observations of this MSP.
Martinez-Medina L, Gieles M, Pichardo B, Peimbert A (2017) New insights in the origin and evolution of the old, metal-rich open cluster NGC 6791, Monthly Notices of the Royal Astronomical Society 474 (1) pp. 32-44 Oxford University Press (OUP)
NGC 6791 is one of the most studied open clusters, it is massive (
I present a new semi-analytic dynamical friction model built upon Chandrasekhar's formalism (Petts et al., 2015, 2016), and its first scientific application regarding the origin of the young stellar populations in the Galactic Centre (Petts and Gualandris, 2017). The model is accurate for spherical potentials of varying inner slope, gamma=[0,2], due to a few key novelties. Firstly, I use physically motivated, radially varying maximum and minimum impact parameters, that describe the range over which interactions are important. Secondly, I use the self-consistent velocity distribution as derived from the distribution function of the galactic potential, including the effect of stars moving faster than satellite. Finally, I reproduce the core-stalling effect seen in simulations of cored galaxies with a ``tidal-stalling'' prescription, which describes when the satellite disrupts the galaxy and forms a steady-state. I implemented dynamical friction analytically in the direct summation N-body code, NBODY6, excellently reproducing the orbital decay of clusters as compared with full N-body models. Since only cluster stars need be modelled in an N-body fashion, my method allows for simulation possibilities that were previously prohibited (e.g. Contenta et al., 2017; Inoue, 2017; Cole et al., 2017).

Using this new method, I explore the scenario in which the young stellar populations in the central parsec of the Milky Way were formed by infalling star clusters. I find that clusters massive enough to reach the central parsec within the lifetime of these populations form very massive stars via collisions. Using up to date - yet conservative - mass loss recipes, I find that these very massive stars lose most of their mass via strong stellar winds, forming large stellar mass black holes incapable of bringing stars to the central parsec. A star cluster infalling in the Galactic Centre within the last 15 Myr would leave an observable population of massive stars from ~1-10 pc, contradicting observations. Thus, I rule out the star cluster inspiral scenario, favouring in-situ formation and/or binary disruption for the origin of the young stars.

Lucas W, Rybak M, Bonnell I, Gieles M (2017) A clustered origin for isolated massive stars, Monthly Notices of the Royal Astronomical Society 474 (3) pp. 3582-3592 Oxford University Press
High-mass stars are commonly found in stellar clusters promoting the idea that their formation occurs due to the physical processes linked with a young stellar cluster. It has recently been reported that isolated high-mass stars are present in the Large Magellanic Cloud. Due to their low velocities it has been argued that these are high-mass stars which formed without a surrounding stellar cluster. In this paper we present an alternative explanation for the origin of these stars in which they formed in a cluster environment but are subsequently dispersed into the field as their natal cluster is tidally disrupted in a merger with a higher-mass cluster. They escape the merged cluster with relatively low velocities typical of the cluster interaction and thus of the larger scale velocity dispersion, similarly to the observed stars. N-body simulations of cluster mergers predict a sizeable population of low velocity (d20 km s?1), high-mass stars at distances of >20 pc from the cluster. High-mass clusters in which gas poor mergers are frequent would be expected to commonly have halos of young stars, including high-mass stars, that were actually formed in a cluster environment.
Star clusters are collisional and dark matter (DM) free stellar systems, where their evolution is ruled by two-body interactions and the galactic potential. Using direct summation N-body simulations, I study how the observational properties of star clusters can be used to: (i) distinguish between DM free and DM dominated objects. From observations, the nature of several faint stellar systems in the Milky Way halo is not clear, therefore, I quantify the contribution of star clusters to the faint stellar systems population. (ii) Probe the underlying DM density of their host galaxy. I apply a new method to the recently discovered Eridanus~II ultra-faint dwarf galaxy that hosts a star cluster in its centre. I find that a cored DM density profile naturally reproduces the observed properties of Eridanus II?s star cluster. (iii) Infer their progenitor properties if they are accreted star clusters, such as Crater. From its properties I find that Crater is likely to be tidally stripped from a dwarf galaxy, and it must have formed extended and with a low concentration. Throughout this thesis, the comparison of simulations and data took into consideration observational biases and uncertainties. I show that the initial conditions of star clusters can heavily influence its present-day properties, and that the stellar evolution prescriptions can also impact the final star cluster properties, such as the neutron stars natal kick distribution. I conclude, through a series of test cases, that N-body simulations can be used to reproduce the observed properties of star clusters, and these can ultimately probe their host galaxy DM distribution.
Gieles M, Zocchi A (2017) Erratum: A family of lowered isothermal models, Monthly Notices of the Royal Astronomical Society 474 (3) pp. 3997-3997 Oxford University Press
This is a correction to: Monthly Notices of the Royal Astronomical Society, Volume 454, Issue 1, 21 November 2015, Pages 576?592, https://doi.org/10.1093/mnras/stv1848
Schneider F, Sana H, Evans C, Bestenlehner J, Castro N, Fossati L, Gräfener G, Langer N, Ramírez-Agudelo O, Sabín-Sanjulián C, Simón-Díaz S, Tramper F, Crowther P, de Koter A, de Mink S, Dufton P, Garcia M, Gieles M, Hénault-Brunet V, Herrero A, Izzard R, Kalari V, Lennon D, Maíz Apellániz J, Markova N, Najarro F, Podsiadlowski P, Puls J, Taylor W, van Loon J, Vink J, Norman C (2018) An excess of massive stars in the local 30 Doradus starburst, Science 359 (6371) pp. 69-71 American Association for the Advancement of Science
The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (Embedded Image). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 Embedded Image and contains 32 ± 12% more stars above 30 Embedded Image than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 Embedded Image, the IMF power-law exponent is Embedded Image, shallower than the Salpeter value of 2.35.
Forbes D, Bastian N, Gieles M, Crain R, Kruijssen J, Larsen S, Ploeckinger S, Agertz O, Trenti M, Ferguson A, Pfeffer J, Gnedin O (2018) Globular Cluster Formation and Evolution in the Context of Cosmological Galaxy Assembly: Open Questions, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 474 (2210) The Royal Society
We discuss some of the key open questions regarding the formation and evolution of globular clusters (GCs) during galaxy formation and assembly within a cosmological framework. The current state-of-the-art for both observations and simulations is described, and we briefly mention directions for future research. The oldest GCs have ages e 12.5 Gyr and formed around the time of reionisation. Resolved colour-magnitude diagrams of Milky Way GCs and direct imaging of lensed proto-GCs at z
Contenta F, Balbinot E, Petts J, Read J, Gieles M, Collins M, Peñarrubia J, Delorme M, Gualandris A (2018) Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II, Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)
We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the `ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results imply that either a cold DM cusp was `heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.
Gieles Mark, Charbonnel C, Krause M, Hénault-Brunet V, Agertz Oscar, Lamers H, Bastian N, Gualandris Alessia, Zocchi A, Petts James (2018) Concurrent formation of supermassive stars and globular clusters:
implications for early self-enrichment,
Monthly Notices of the Royal Astronomical Society 478 (2) sty1059 pp. 2461-2479 Oxford University Press
We present a model for the concurrent formation of globular clusters (GCs) and supermassive stars (SMSs, >103M™) to address the origin of the HeCNONaMgAl abundance anomalies in GCs. GCs form in converging gas flows and accumulate low-angular momentum gas, which accretes onto protostars. This leads to an adiabatic contraction of the cluster and an increase of the stellar collision rate. A SMS can form via runaway collisions if the cluster reaches sufficiently high density before two-body relaxation halts the contraction. This condition is met if the number of stars s106 and the gas accretion rate s105M™/Myr, reminiscent of GC formation in high gas-density environments, such as -- but not restricted to -- the early Universe. The strong SMS wind mixes with the inflowing pristine gas, such that the protostars accrete diluted hot-hydrogen burning yields of the SMS. Because of continuous rejuvenation, the amount of processed material liberated by the SMS can be an order of magnitude higher than its maximum mass. This `conveyor-belt' production of hot-hydrogen burning products provides a solution to the mass budget problem that plagues other scenarios. Additionally, the liberated material is mildly enriched in helium and relatively rich in other hot-hydrogen burning products, in agreement with abundances of GCs today. Finally, we find a super-linear scaling between the amount of processed material and cluster mass, providing an explanation for the observed increase of the fraction of processed material with GC mass. We discuss open questions of this new GC enrichment scenario and propose observational tests.
Zocchi A, Gieles M, Hénault-Brunet V (2018) The effect of stellar-mass black holes on the central kinematics of É Cen: a cautionary tale for IMBH interpretations, Monthly Notices of the Royal Astronomical Society Oxford University Press
The search for intermediate-mass black holes (IMBHs) in the centre of globular clusters is
often based on the observation of a central cusp in the surface brightness profile and a rise
towards the centre in the velocity dispersion profiles. Similar signatures, however, could result
from other effects, that need to be taken into account in order to determine the presence (or
the absence) of an IMBH in these stellar systems. Following our previous exploration of the
role of radial anisotropy in shaping these observational signatures, we analyse here the effects
produced by the presence of a population of centrally concentrated stellar-mass black holes.
We fit dynamical models to É Cen data, and we show that models with ~ 5% of their mass
in black holes (consistent with ~ 100% retention fraction after natal kicks) can reproduce the
data.When simultaneously considering both radial anisotropy and mass segregation, the bestfit
model includes a smaller population of remnants, and a less extreme degree of anisotropy
with respect to the models that include only one of these features. These results underline that
before conclusions about putative IMBHs can be made, the effects of stellar-mass black holes
and radial anisotropy need to be properly accounted for.
Schneider F.R.N., Ramírez-Agudelo O.H., Tramper F., Bestenlehner J.M., Castro N., Sana H., Evans C.J., Sabín-Sanjulián C., Simón-Díaz S., Langer N., Fossati L., Gräfener G., Crowther P.A., de Mink S.E., de Koter A., Gieles M., Herrero A., Izzard R.G., Kalari V., Klessen R.S., Lennon D.J., Mahy L., Maíz Apellániz J., Markova N., van Loon J.Th., Vink J.S., Walborn N.R. (2018) The VLT-FLAMES Tarantula Survey. XXIX. Massive star formation in the local 30 Doradus starburst, Astronomy and Astrophysics EDP Sciences
The 30 Doradus (30 Dor) nebula in the Large Magellanic Cloud (LMC) is the brightest HII region in the Local Group and a prototype
starburst similar to those found in high redshift galaxies. It is thus a stepping stone to understand the complex formation processes of
stars in starburst regions across the Universe. Here, we have studied the formation history of massive stars in 30 Dor using masses and
ages derived for 452 mainly OB stars from the spectroscopic VLT-FLAMES Tarantula Survey (VFTS). We find that stars of all ages
and masses are scattered throughout 30 Dor. This is remarkable because it implies that massive stars either moved large distances or
formed independently over the whole field of view in relative isolation. We find that both channels contribute to the 30 Dor massive
star population. Massive star formation rapidly accelerated about 8 Myr ago, first forming stars in the field before giving birth to the
stellar populations in NGC 2060 and NGC 2070. The R136 star cluster in NGC 2070 formed last and, since then, about 1 Myr ago,
star formation seems to be diminished with some continuing in the surroundings of R136. Massive stars within a projected distance
of 8 pc of R136 are not coeval but show an age range of up to 6 Myr. Our mass distributions are well populated up to 200M˜. The
inferred IMF is shallower than a Salpeter-like IMF and appears to be the same across 30 Dor. By comparing our sample of stars to
stellar models in the Hertzsprung?Russell diagram, we find evidence for missing physics in the models above log L=L˜ = 6 that is
likely connected to enhanced wind mass loss for stars approaching the Eddington limit. Our work highlights the key information about
the formation, evolution and final fates of massive stars encapsulated in the stellar content of 30 Dor, and sets a new benchmark for
theories of massive star formation in giant molecular clouds.
Terlevich Elena, Fernández-Arenas David, Terlevich Roberto, Gieles Mark, Chávez Ricardo, González-Morán Ana Luisa (2018) From Giant H ii regions and H ii galaxies to globular clusters and compact dwarf ellipticals, Monthly Notices of the Royal Astronomical Society sty2325 Oxford University Press
Massive starforming regions like Giant H ii Regions (GHIIR) and H ii Galaxies (HIIG) are emission line systems ionized by compact young massive star clusters (YMC) with masses ranging from 104M™ to 108M™. We model the photometric and dynamical evolution over a Hubble time of the massive gravitationally bound systems that populate the tight relation between absolute blue magnitude and velocity dispersion (MB ? Ã) of GHIIR and HIIG and compare the resulting relation with that one of old stellar systems: globular clusters, elliptical galaxies, bulges of spirals. After 12 Gyr of evolution their position on the à vs. MB plane coincides ? depending on the initial mass ? either with the globular clusters for systems with initial mass M 106M™. The slope change in the MB ? à and MB-size relations at cluster masses around 106M™ is due to the larger impact of the dynamical evolution on the lower mass clusters. We interpret our result as an indication that the YMC that ionize GHIIR and HIIG can evolve to form globular clusters and ultra compact dwarf ellipticals in about 12 Gyr so that present day globular clusters and ultra compact dwarf ellipticals may have formed in conditions similar to those observed in today GHIIR and HIIG.