The Tidal Disruption of Giant Stars and Their Contribution to the Flaring Supermassive Black Hole Population

TL;DR

This study uses hydrodynamic simulations to explore how giant stars are tidally disrupted by supermassive black holes, revealing unique flare signatures and the influence of stellar evolution on disruption rates.

Contribution

First hydrodynamic simulations of giant star disruptions showing core influence on survival and flare characteristics, with implications for understanding SMBH feeding and stellar demographics.

Findings

Giant star disruptions produce long-lasting, rising brightness flares.

Disruption rates depend on stellar evolution and SMBH mass, especially near 10^8 solar masses.

Evolved stars dominate flares beyond the SMBH mass limit, affecting observed event rates.

Abstract

Sun-like stars are thought to be regularly disrupted by supermassive black holes (SMBHs) within galactic nuclei. Yet, as stars evolve off the main sequence their vulnerability to tidal disruption increases drastically as they develop a bifurcated structure consisting of a dense core and a tenuous envelope. Here we present the first hydrodynamic simulations of the tidal disruption of giant stars and show that the core has a substantial influence on the star's ability to survive the encounter. Stars with more massive cores retain large fractions of their envelope mass, even in deep encounters. Accretion flares resulting from the disruption of giant stars should last for tens to hundreds of years. Their characteristic signature in transient searches would not be the $t^{-5/3}$ decay typically associated with tidal disruption events, but a correlated rise over many orders of magnitude in brightness on months to years timescales. We calculate the relative disruption rates of stars of varying evolutionary stages in typical galactic centers, then use our results to produce Monte Carlo realizations of the expected flaring event populations. We find that the demographics of tidal disruption flares are strongly dependent on both stellar and black hole mass, especially near the limiting SMBH mass scale of $\sim 10^8 M_\odot$. At this black hole mass, we predict a sharp transition in the SMBH flaring diet beyond which all observable disruptions arise from evolved stars, accompanied by a dramatic cutoff in the overall tidal disruption flaring rate. Black holes less massive than this limiting mass scale will show observable flares from both main sequence and evolved stars, with giants contributing up to 10% of the event rate. The relative fractions of stars disrupted at different evolutionary states can constrain the properties and distributions of stars in galactic nuclei other than our own.