Stellar Revival and Repeated Flares in Deeply Plunging Tidal Disruption Events

TL;DR

This paper challenges the traditional view that deep tidal disruption events fully destroy stars, showing instead that stellar cores can reform after such encounters, which has implications for observed repeating nuclear transients.

Contribution

It demonstrates that stars can partially survive deep tidal disruptions and reform their cores, contradicting the assumption of complete destruction at high encounter parameter values.

Findings

Stellar cores can reform after deep tidal disruptions.

Reformed cores comprise about 25% of the original stellar mass.

Core formation occurs on a bound orbit around the black hole.

Abstract

Tidal disruption events with tidal radius $r_{\rm t}$ and pericenter distance $r_{\rm p}$ are characterized by the quantity $\beta = r_{\rm t}/r_{\rm p}$, and "deep encounters" have $\beta \gg 1$. It has been assumed that there is a critical $\beta \equiv \beta_{\rm c} \sim 1$ that differentiates between partial and full disruption: for $\beta < \beta_{\rm c}$ a fraction of the star survives the tidal interaction with the black hole, while for $\beta > \beta_{\rm c}$ the star is completely destroyed, and hence all deep encounters should be full. Here we show that this assumption is incorrect by providing an example of a $\beta = 16$ encounter between a $\gamma = 5/3$, solar-like polytrope and a $10^6 M_{\odot}$ black hole -- for which previous investigations have found $\beta_{\rm c} \simeq 0.9$ -- that results in the reformation of a stellar core post-disruption that comprises approximately 25\% of the original stellar mass. We propose that the core reforms under self-gravity, which remains important because of the compression of the gas both near pericenter, where the compression occurs out of the orbital plane, and substantially after pericenter, where compression is within the plane. We find that the core forms on a bound orbit about the black hole, and we discuss the corresponding implications of our findings in the context of recently observed, repeating nuclear transients.