Partial, zombie, and full tidal disruption of stars by supermassive black holes

TL;DR

This paper presents detailed simulations of tidal disruption events of stars by supermassive black holes, revealing different fallback rate behaviors, core reformation, and debris dynamics, with implications for understanding these cosmic phenomena.

Contribution

It introduces new simulation results showing the distinct fallback rate indices for partial and full disruptions, core reformation after destruction, and a novel analytical fallback rate fitting function.

Findings

Full disruptions have a fallback rate index of -5/3.

Partial disruptions exhibit a fallback rate index of -9/4.

Self-gravity influences debris evolution and core re-collapse.

Abstract

We present long-duration numerical simulations of the tidal disruption of stars modelled with accurate stellar structures and spanning a range of pericentre distances, corresponding to cases where the stars are partially and completely disrupted. We substantiate the prediction that the late-time power-law index of the fallback rate $n_{\infty} \simeq -5/3$ for full disruptions, while for partial disruptions---in which the central part of the star survives the tidal encounter intact---we show that $n_{\infty} \simeq -9/4$. For the subset of simulations where the pericenter distance is close to that which delineates full from partial disruption, we find that a stellar core can reform after the star has been completely destroyed; for these events the energy of the zombie core is slightly positive, which results in late-time evolution from $n \simeq -9/4$ to $n \simeq -5/3$. We find that self-gravity can generate an $n(t)$ that deviates from $n_{\infty}$ by a small but significant amount for several years post-disruption. In one specific case with the stellar pericenter near the critical value, we find self-gravity also drives the re-collapse of the central regions of the debris stream into a collection of several cores while the rest of the stream remains relatively smooth. We also show that it is possible for the surviving stellar core in a partial disruption to acquire a circumstellar disc that is shed from the rapidly rotating core. Finally, we provide a novel analytical fitting function for the fallback rates that may also be useful in a range of contexts beyond TDEs.