Finite, Intense Accretion Bursts from Tidal Disruption of Stars on Bound Orbits

TL;DR

This study uses 3D simulations to show that relativistic effects cause intense, rapid accretion bursts from tidally disrupted stars on bound orbits, deviating from traditional models.

Contribution

It demonstrates the critical role of relativistic precession in debris circularization and accretion rate enhancement for stars on bound orbits around supermassive black holes.

Findings

Accretion occurs in finite time for high but sub-critical eccentricities.

Relativistic precession causes debris stream crossings and rapid circularization.

Accretion rates can greatly exceed the Eddington limit, deviating from the canonical $t^{-5/3}$ fallback rate.

Abstract

We study accretion processes for tidally disrupted stars approaching supermassive black holes on bound orbits, by performing three dimensional Smoothed Particle Hydrodynamics simulations with a pseudo-Newtonian potential. We find that there is a critical value of the orbital eccentricity below which all the stellar debris remains bound to the black hole. For high but sub-critical eccentricities, all the stellar mass is accreted onto the black hole in a finite time, causing a significant deviation from the canonical $t^{-5/3}$ mass fallback rate. When a star is on a moderately eccentric orbit and its pericenter distance is deeply inside the tidal disruption radius, there can be several orbit crossings of the debris streams due to relativistic precession. This dissipates orbital energy in shocks, allowing for rapid circularization of the debris streams and formation of an accretion disk. The resultant accretion rate greatly exceeds the Eddington rate and differs strongly from the canonical rate of $t^{-5/3}$. By contrast, there is little dissipation due to orbital crossings for the equivalent simulation with a purely Newtonian potential. This shows that general relativistic precession is crucial for accretion disk formation via circularization of stellar debris from stars on moderately eccentric orbits.