Based on Norman, Nixon & Coughlin (2021, ApJ 923 184)

This research, led by undergraduate student Sarah Norman, presents numerical simulations of extreme encounters of stars with supermassive black holes. When a star gets sufficiently close to a supermassive black hole, the tidal field of the black hole can overwhelm the star’s self-gravity leading to the star being pulled apart into a thin stream of debris. Some of this debris falls back to the black hole over a period of order a year, powering a luminous flare that we can observe in distant galaxies. These events occur approximately every 10,000 years in each galaxy, and the number of observed events is expected to increase dramatically over the next few years due to new observing facilities, such as the Rubin Observatory, that will scan the sky every few days to find these energetic transients.

The numerical simulations presented in Norman et al. (2021), performed on DiRAC’s Data Intensive at Leicester (DIaL) cluster, explore several aspects of the physics of tidal disruption events including: (1) the pericentre distance of the stellar orbit, (2) the effects of heating due to shocks as the star is strongly compressed by the black hole’s gravitational field, and (3) the shape and time-dependence of the energy distribution of the stellar debris. We also varied the numerical resolution of the simulations, varying the SPH particle number from 250k to 128M, to determine the level of convergence of the simulation results. With these simulations we showed that simulating tidal disruptions with sufficiently small pericentre distance requires higher resolution than is typically employed in these calculations, with > 10 million particles required to accurately determine the orbital properties of the stellar debris. By comparing simulations that included and excluded the heating due to shocks we were able to show that this effect is only important for extreme encounters with small pericentre distances, and further, by comparing our simulations with detailed analytical calculations we were able to show that any shocks which do form are limited to Mach numbers of order unity. Finally, our results show that, while the shape of the energy distribution of the stellar debris is dependent on the original stellar orbit, the breadth of the energy distribution is essentially unchanged for the different orbits we simulated.

These simulations, and the analysis presented in Norman et al. (2021), has advanced our understanding of the physics of tidal disruption events, and the numerical parameters required to simulate them with high accuracy. Modelling observational data of these events can provide information on a number of important astrophysical topics including stellar dynamics in galaxy centres, supermassive black hole demographics, accretion physics and radiative processes. The next few years will see a substantial increase in the number of observed events and it is important that our theoretical and numerical calculations are robust enough to facilitate detailed comparisons in this area.

Categories: 2021 Highlights