Seminars

We host a mix of virtual and hybrid seminars every other Thursday from 1:00-2:00pm.  For information on how to join them, or for any other queries regarding the seminars, please contact Dr Jason Hunt or Dr Eugene Vasiliev.

Most seminars are in Surrey space centre room 13.

2025/26

11th September 2025: Shashank Dattathri (Yale) - in person

Core dynamics and instability: an astrophysical laboratory of dark matter.
Numerical simulations have shown that dynamical friction is highly suppressed within galaxies and dark matter halos with central density cores, a phenomenon known as core stalling. Within the core region, massive perturbers experience an outward push, which is termed as dynamical buoyancy. These phenomena may inhibit the inspiral of black holes and other massive objects, and therefore may have profound implications for galaxy and black hole evolution. In this talk, I present new results regarding the origin and mechanism of core stalling and dynamical buoyancy. I show that ultimately, it is the overall shape of the system's distribution function that determines the strength of dynamical friction, and plateaus in the distribution function give rise to stalling. If the distribution function has an inflection, which occurs when the transition from the outer region to the core is too rapid, the system is subject to a dipole instability, which is closely linked to buoyancy. Finally, I compare how these phenomena manifest in cold dark matter versus self-interacting dark matter halos, which naturally form cores. I conclude with a discussion on constraining the nature of dark matter by using observations of massive objects such as black holes, globular clusters, and nuclear star clusters in the central regions of galaxies.


 

 

 

25th September: Mauro Cabrera-Gadea - in person

The warp of the Milky Way disc traced by Cepheids and RR Lyrae.

We know since the late 1950s that the Milky Way disc is warped, but it’s origin remains unknown. To better understand it’s origin, we should characterize its structure and kinematics. For this reason we have characterize the warp with Classical Cepheids and RR Lyrae. For the case of Cepheids we have analyze the vertical height (Z) and vertical velocity (Vz) via Fourier decomposition without any assumptions in our model, reveling  an asymmetric signal in both variables. 

For the first time, we find that RR Lyrae from the thin disk traces out a warped disc, suggesting that both populations with different ages trace out a similarly warped thin disc. We measure the mean Z and Vz for those RR Lyrae in the anticentre and find differences with the mean traced by Cepheids.

Finally, a joint analysis of the Z and Vz allows us to derive the pattern speed of the warp for the Cepheids and RR Lyrae. Applying it to our results for the Cepheids, we find, for the m=1 mode, a constant angular velocity in the stellar rotation direction of 9.2+-3.1 km/s/kpc, a null amplitude variation up to 14 kpc, and a slight increase at larger radii, in agreement with previous work. For the RR Lyrae, we estimate the pattern speed of their warp and found agreement with the trend and values found with Cepheids.

This new window into the analysis of the warp with different standard candles will help us constrain its origin, because its mechanism of formation must account for the differences found between the two populations.

9th October: Umberto Battino (University of Naples) - in person

Trans-Fe elements from type Ia supernovae.
A Type Ia supernova (SNIa) marks the catastrophic explosion of a white dwarf in a binary system. These events play a crucial role in galactic chemical evolution and serve as pivotal standardizable candles for measuring cosmic distances, underpinning the discovery of the Universe's accelerated expansion. However, the progenitors of SNIa remain uncertain, introducing challenges to their use in cosmology and nucleosynthesis predictions.

In this work, we present a grid of five models detailing the evolution and nucleosynthesis of slowly merging carbon-oxygen white dwarfs approaching the Chandrasekhar mass. These models test a variety of physics input settings, including accretion rates, nuclear reaction rates, convection parameters, and the composition of the accreted material. During the merger process, as the mass of the primary white dwarf approaches the Chandrasekhar limit, carbon burning is initiated first on the surface before eventually igniting explosively at the center. As a consequence, the 22Ne(a,n)25Mg reaction activates in the outer layers of all models, producing a weak s-process-like abundance pattern peaking at Kr, which is overproduced by more than a factor of ~1000 compared to solar. The trans-Fe elements-enriched outer layer mass varies from ~0.04Msun to ~0.11Msun, depending on the accretion rate. Additionally, up to 6 10^-6 Msun of 60Fe are produced in the same outer layers. Our explosion simulations of these progenitor models eject significant amount of first-peak elements (e.g., Kr, Sr) and light p-nuclei (e.g., 74Se).

In a previous theoretical study, we found that a similar nucleosynthesis process during the progenitor phase may also occur on the surface of near-Chandrasekhar white dwarfs formed through the accretion of H-rich material via the single-degenerate scenario. Therefore, these results suggest trans-Fe enrichment might be a hallmark of near-Chandrasekhar SNIa ejecta, regardless of the specific progenitor channel, and could provide a new spectral signature distinguishing them from sub-Chandrasekhar explosions.

23rd October: Alexander Rawlings (University of Helsinki) - online

6th November: Amanda Byström (University of Edinburgh) - in person

4th December: Hanyuan Zhang (University of Cambridge) - in person

 

4th June 2026: Stephen Wilkins (University of Sussex) - in person

 

 

 

Past seminars