While many intermediate- and high-mass main sequence stars are rapidly and differentially rotating, the effects of rotation on oscillation modes are poorly known. In this communication we present a first study of axisymmetric gravito-inertial modes in the radiative zone of a differentially rotating star. We consider a simplified model where the radiative zone of the star is a linearly stratified rotating fluid within a spherical shell, with differential rotation due to baroclinic effects. We solve the eigenvalue problem with high-resolution spectral computations and determine the propagation domain of the waves through the theory of characteristics. We explore the propagation properties of two kinds of modes: those that can propagate in the entire shell and those that are restricted to a sub-domain. Some of the modes that we find concentrate kinetic energy around short-period shear layers known as attractors. We describe various geometries for the propagation domains, conditioning the surface visibility of the corresponding modes.
Antoci V, Cunha M S, Bowman D M, Murphy S J, Kurtz D W, Bedding T R, Borre C C, Christophe S, Daszyńska-Daszkiewicz J, Fox-Machado L, Hernández A García, Ghasemi H, Handberg R, Johnston C, Hansen H, Hasanzadeh A, Houdek G, Justesen A B, Alicavus F Kahraman, Kotysz K, Latham D, Matthews J M, Mønster J, Pigulski A, Niemczura E, Paunzen E, Arias J P Sánchez, Pepper J, Richey-Yowell T, Safari H, Seager S, Smalley B, Shutt T, Wu T, Sódor A, Suárez J-C, Tkachenko A, Zwintz K, Forteza S Barceló, Brunsden E, Bognár Z, Buzasi D L, Chowdhury S, Guzik J A, De Cat P, Evans J A, Guo Z, Jevtic N, Lampens P, Martiz M Lares, Lovekin C, Li G, Nemec J M, Mirouh G M, Mkrtichian D, Monteiro M J P F G, Ouazzani R-M, Pascual-Granado J, Reese D R, Rieutord M, Rodon J R, Skarka M, Weiss W W, Sowicka P, Stateva I, Szabó R (2019)The first view of δ Scuti and γ Doradus stars with the TESS mission, In: Monthly Notices of the Royal Astronomical Society
Oxford University Press (OUP)
We present the first asteroseismic results for δ Scuti and γ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilise the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung-Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, γ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74% for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young δ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler.
Balona L A, Handler G, Chowdhury S, Ozuyar D, Engelbrecht C A, Mirouh G M, Wade G A, David-Uraz A, Cantiello M (2019)Rotational modulation in TESS B stars, In: Monthly Notices of the Royal Astronomical Society
Oxford University Press (OUP)
Light curves and periodograms of 160 B stars observed by the TESS space mission and 29 main-sequence B stars from Kepler and K2 were used to classify the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 percent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.
Pedersen May G., Chowdhury Sowgata, Johnston Cole, Bowman Dominic M., Aerts Conny, Handler Gerald, Cat Peter De, Neiner Coralie, David-Uraz Alexandre, Buzasi Derek, Tkachenko Andrew, Simón-Díaz Sergio, Moravveji Ehsan, Sikora James, Mirouh Giovanni, Lovekin Catherine C., Vanderspek Roland K., Cantiello Matteo, Daszyńska-Daszkiewicz Jadwiga, Pigulski Andrzej, Ricker George R. (2019)Diverse Variability of O and B Stars Revealed from 2-minute Cadence Light Curves in Sectors 1 and 2 of the TESS Mission: Selection of an Asteroseismic Sample, In: The Astrophysical Journal872(1)
Uncertainties in stellar structure and evolution theory are largest for stars undergoing core convection on the main sequence. A powerful way to calibrate the free parameters used in the theory of stellar interiors is asteroseismology, which provides direct measurements of angular momentum and element transport. We report the detection and classification of new variable O and B stars using high-precision short-cadence (2 minutes) photometric observations assembled by the Transiting Exoplanet Survey Satellite (TESS). In our sample of 154 O and B stars, we detect a high percentage (90%) of variability. Among these we find 23 multiperiodic pulsators, 6 eclipsing binaries, 21 rotational variables, and 25 stars with stochastic low-frequency variability. Several additional variables overlap between these categories. Our study of O and B stars not only demonstrates the high data quality achieved by TESS for optimal studies of the variability of the most massive stars in the universe, but also represents the first step toward the selection and composition of a large sample of O and B pulsators with high potential for joint asteroseismic and spectroscopic modeling of their interior structure with unprecedented precision.
Handler Gerald, Pigulski Andrzej, Daszyńska-Daszkiewicz Jadwiga, Irrgang Andreas, Kilkenny David, Guo Zhao, Przybilla Norbert, Aliçavuş Filiz Kahraman, Kallinger Thomas, Pascual-Granado Javier, Niemczura Ewa, Różański Tomasz, Chowdhury Sowgata, Buzasi Derek L., Mirouh Giovanni M., Bowman Dominic M., Johnston Cole, Pedersen May G., De Cat Peter, Simón-Díaz Sergio, Moravveji Ehsan, Gazeas Kosmas, Vanderspek Roland K., Ricker George R. (2019)Asteroseismology of Massive Stars with the TESS Mission: The Runaway β Cep Pulsator PHL 346 = HN Aqr, In: The Astrophysical Journal Letters873(1)
We report an analysis of the first known β Cep pulsator observed by the Transiting Exoplanet Survey Satellite (TESS) mission, the runaway star PHL 346 = HN Aqr. The star, previously known as a singly periodic pulsator, has at least 34 oscillation modes excited, 12 of those in the g-mode domain and 22 p modes. Analysis of archival data implies that the amplitude and frequency of the dominant mode and the stellar radial velocity were variable over time. A binary nature would be inconsistent with the inferred ejection velocity from the Galactic disk of 420 km s−1, which is too large to be survivable by a runaway binary system. A kinematic analysis of the star results in an age constraint (23 ± 1 Myr) that can be imposed on asteroseismic modeling and that can be used to remove degeneracies in the modeling process. Our attempts to match the excitation of the observed frequency spectrum resulted in pulsation models that were too young. Hence, asteroseismic studies of runaway pulsators can become vital not only in tracing the evolutionary history of such objects, but to understand the interior structure of massive stars in general. TESS is now opening up these stars for detailed asteroseismic investigation.
Oscillation modes in fast-rotating stars can be split into several subclasses, each with their own properties. To date, seismology of these stars cannot rely on regular pattern analysis and scaling relations. However, recently there has been the promising discovery of large separations observed in spectra of fast-rotating δ Scuti stars; they were attributed to the island-mode subclass, and linked to the stellar mean density through a scaling law. In this work, we investigate the relevance of this scaling relation by computing models of fast-rotating stars and their oscillation spectra. In order to sort the thousands of oscillation modes thus obtained, we train a convolutional neural network isolating the island modes with 96 per cent accuracy. Arguing that the observed large separation is systematically smaller than the asymptotic one, we retrieve the observational Δν--ρ¯¯¯ scaling law. This relation will be used to drive forward modelling efforts, and is a first step towards mode identification and inversions for fast-rotating stars.
Early-type stars generally tend to be fast rotators. In these stars, mode identification is very challenging as the effects of rotation are not well known. We consider here the example of α Ophiuchi, for which dozens of oscillation frequencies have been measured. We model the star using the two-dimensional structure code ESTER, and we compute both adiabatic and non-adiabatic oscillations using the TOP code. Both calculations yield very complex spectra, and we used various diagnostic tools to try and identify the observed pulsations. While we have not reached a satisfactory mode-to-mode identification, this paper presents promising early results.
Oscillations have been detected in a variety of stars, including intermediate- and high-mass main sequence stars. While many of these stars are rapidly and differentially rotating, the effects of rotation on oscillation modes are poorly known. In this communication we present a first study on axisymmetric gravito-inertial modes in the radiative zone of a differentially rotating star. These modes probe the deep layers of the star around its convective core. We consider a simplified model where the radiative zone of a star is a linearly stratified rotating fluid within a spherical shell, with differential rotation due to baroclinic effects. We solve the eigenvalue problem with high-resolution spectral simulations and determine the propagation domain of the waves through the theory of characteristics. We explore the propagation properties of two kinds of modes: those that can propagate in the entire shell and those that are restricted to a subdomain. Some of the modes that we find concentrate kinetic energy around short-period shear layers known as attractors. We characterise these attractors by the dependence of their Lyapunov exponent with the ____BV frequency of the background and the oscillation frequency of the mode. Finally, we note that, as modes associated with short-period attractors form dissipative structures, they could play an important role for tidal interactions but should be dismissed in the interpretation of observed oscillation frequencies.
Holschneider M., Lignières F., Petit P., Rainer M., Paletou F., Wade G., Alecian E., Carfantan H., Blazère A., Mirouh G. M., Böhm T. (2015)Discovery of starspots on Vega, In: Astronomy & Astrophysics577A64
Context. The theoretically studied impact of rapid rotation on stellar evolution needs to be compared with these results of high-resolution spectroscopy-velocimetry observations. Early-type stars present a perfect laboratory for these studies. The prototype A0 star Vega has been extensively monitored in recent years in spectropolarimetry. A weak surface magnetic field was detected, implying that there might be a (still undetected) structured surface. First indications of the presence of small amplitude stellar radial velocity variations have been reported recently, but the confirmation and in-depth study with the highly stabilized spectrograph SOPHIE/OHP was required.
Aims. The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order to reveal potential activity tracers, exoplanet companions, and stellar oscillations.
Methods. Vega was monitored in quasi-continuous high-resolution echelle spectroscopy with the highly stabilized velocimeter SOPHIE/OHP. A total of 2588 high signal-to-noise spectra was obtained during 34.7 h on five nights (2 to 6 of August 2012) in high-resolution mode at R = 75 000 and covering the visible domain from 3895−6270 Å. For each reduced spectrum, least square deconvolved equivalent photospheric profiles were calculated with a Teff = 9500 and log g = 4.0 spectral line mask. Several methods were applied to study the dynamic behaviour of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others).
Results. We present the discovery of a spotted stellar surface on an A-type standard star (Vega) with very faint spot amplitudes ΔF/Fc ~ 5 × 10-4. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, unambiguously confirming the results of previous spectropolarimetric studies. Most of these brightness inhomogeneities seem to be located in lower equatorial latitudes. Either a very thin convective layer can be responsible for magnetic field generation at small amplitudes, or a new mechanism has to be invoked to explain the existence of activity tracing starspots. At this stage it is difficult to disentangle a rotational from a stellar pulsational origin for the existing higher frequency periodic variations.
Conclusions. This first strong evidence that standard A-type stars can show surface structures opens a new field of research and ask about a potential link with the recently discovered weak magnetic field discoveries in this category of stars.
The gravito-inertial waves propagating over a shellular baroclinic flow inside a rotating spherical shell are analysed using the Boussinesq approximation. The wave properties are examined by computing paths of characteristics in the non-dissipative limit, and by solving the full dissipative eigenvalue problem using a high-resolution spectral method. Gravito-inertial waves are found to obey a mixed-type second-order operator and to be often focused around short-period attractors of characteristics or trapped in a wedge formed by turning surfaces and boundaries. We also find eigenmodes that show a weak dependence with respect to viscosity and heat diffusion just like truly regular modes. Some axisymmetric modes are found unstable and likely destabilized by baroclinic instabilities. Similarly, some non-axisymmetric modes that meet a critical layer (or corotation resonance) can turn unstable at sufficiently low diffusivities. In all cases, the instability is driven by the differential rotation. For many modes of the spectrum, neat power laws are found for the dependence of the damping rates with diffusion coefficients, but the theoretical explanation for the exponent values remains elusive in general. The eigenvalue spectrum turns out to be very rich and complex, which lets us suppose an even richer and more complex spectrum for rotating stars or planets that own a differential rotation driven by baroclinicity.
Context. As the global magnetic field of the Sun has an activity cycle, one expects to observe some variation of the dynamical properties of the flows visible in the photosphere.
Aims. We investigate the flow field during the solar cycle by analysing SDO/HMI observations of continuum intensity, Doppler velocity and longitudinal magnetic field.
Methods. We first picked data at disk center during 6 yr along the solar cycle with a 48-h time step in order to study the overall evolution of the continuum intensity and magnetic field. Then we focused on thirty 6-h sequences of quiet regions without any remnant of magnetic activity separated by 6 months, in summer and winter, when disk center latitude B0 is close to zero. The horizontal velocity was derived from the local correlation tracking technique over a field of view of 216.4 Mm × 216.4 Mm located at disk center.
Results. Our measurements at disk center show the stability of the flow properties between meso- and supergranular scales along the solar cycle.
Conclusions. The network magnetic field, produced locally at disk center independently from large scale dynamo, together with continuum contrast, vertical and horizontal flows, seem to remain constant during the solar cycle.