[Paper Review] Partial, zombie, and full tidal disruption of stars by supermassive black holes

C. J. Nixon, Eric R. Coughlin+1 more|arXiv (Cornell University)|Aug 9, 2021

Astrophysical Phenomena and ObservationsPhysics and Astronomy124 references43 citations

TL;DR

This study presents long-duration hydrodynamical simulations of tidal disruption events (TDEs) involving stars with realistic stellar structures, revealing that full disruptions yield a late-time fallback rate power-law index of n∞ ≃ −5/3, while partial disruptions yield n∞ ≃ −9/4. For disruptions near the critical pericentre, debris can re-collapse into a 'zombie' core with slightly positive energy, causing the fallback index to evolve from −9/4 to −5/3 over ~100 years, highlighting the importance of self-gravity and long-term evolution in TDEs.

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.

Motivation & Objective

• To investigate the late-time fallback rate evolution in tidal disruption events (TDEs) with accurate stellar structures.
• To determine how self-gravity and stellar structure influence the power-law index of the fallback rate.
• To explore the formation and evolution of 'zombie' cores in near-critical TDEs where the star is fully disrupted but debris re-collapses.
• To examine the potential for circumstellar disc formation around surviving stellar cores in partial TDEs.
• To develop a novel analytical fitting function for TDE fallback rates applicable beyond TDEs.

Proposed method

• Conducted long-duration, 3D hydrodynamical simulations using the smoothed particle hydrodynamics (SPH) code SPHINX with accurate stellar profiles from the MESA stellar evolution code.
• Simulated TDEs with stars of 1M⊙ ZAMS, 1M⊙ MAMS, and 0.3M⊙ MAMS on parabolic orbits with varying pericentre distances (β ≈ 1.0–3.0).
• Used a novel analytical fitting function (equation 2) to model fallback rates with few parameters, enabling robust comparison with simulation data.
• Tracked the fallback rate and its instantaneous power-law index n(t) over timescales exceeding 100 years to capture late-time evolution.
• Analyzed the role of self-gravity in driving re-collapse of debris into one or multiple cores near the critical disruption threshold.
• Quantified the energy of surviving cores relative to the black hole to assess their long-term orbital fate and fallback evolution.

Experimental results

Research questions

• RQ1 What is the late-time power-law index of the fallback rate for full versus partial tidal disruptions?
• RQ2 Can debris from a fully disrupted star re-collapse into a bound core ('zombie' core) when the pericentre is near the critical disruption distance?
• RQ3 How does self-gravity influence the fallback rate evolution in TDEs, particularly for systems near the disruption threshold?
• RQ4 Can a surviving stellar core in a partial TDE acquire a circumstellar disc, and what is its origin and longevity?
• RQ5 Can a simple analytical function accurately fit TDE fallback rate light curves across different disruption regimes?

Key findings

• For full tidal disruptions, the fallback rate power-law index asymptotically approaches n∞ ≃ −5/3, consistent with classical theory.
• For partial disruptions, where a stellar core survives intact, the late-time fallback index is n∞ ≃ −9/4, independent of the core mass fraction.
• In near-critical disruptions (β ≈ βc), debris can re-collapse into a single 'zombie' core with slightly positive energy, causing n(t) to evolve from ≈−9/4 to ≈−5/3 over ~100 years.
• For β ≈ 1.61 and 1.70, the fallback rate evolution shows a transition from n ≈ −9/4 to n ≈ −5/3, with the timescale of transition matching theoretical estimates of Tϵ ≈ 87–190 years.
• Self-gravity significantly affects fallback rates even for β up to twice the critical value, particularly in low-mass stars (0.3M⊙ MAMS), where it drives re-collapse and alters the fallback profile.
• Surviving stellar cores in partial TDEs can acquire circumstellar discs via equatorial ejection from rapidly rotating cores and sustained feeding from the bound debris stream, potentially leading to observable disc emission.