[Paper Review] Disc formation from stellar tidal disruptions

Clément Bonnerot, Elena M. Rossi+2 more|arXiv (Cornell University)|Jan 19, 2015

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper uses hydrodynamical simulations to show that stellar debris from tidal disruptions circularizes on an orbital timescale due to relativistic apsidal precession, leading to stream self-crossing. If cooling is efficient, a narrow, super-Eddington accreting ring forms; if cooling is suppressed, a centrifugally supported torus forms that drains at super-Eddington rates during circularization.

ABSTRACT

The potential of tidal disruption of stars to probe otherwise quiescent supermassive black holes cannot be exploited, if their dynamics is not fully understood. So far, the observational appearance of these events has been commonly derived from analytical extrapolations of the debris dynamical properties just after the stellar disruption. In this paper, we perform hydrodynamical simulations of stars in highly eccentric orbits, that follow the stellar debris after disruption and investigate their ultimate fate. We demonstrate that gas debris circularize on an orbital timescale because relativistic apsidal precession causes the stream to self-cross. The higher the eccentricity and/or the deeper the encounter, the faster is the circularization. If the internal energy deposited by shocks during stream self-interaction is readily radiated, the gas forms a narrow ring at the circularization radius. It will then proceed to accrete viscously at a super-Eddington rate, puffing up under radiation pressure. If instead cooling is impeded, the gas forms an extended, mostly centrifugally supported torus. In this case, however, the viscous timescale is comparable to the circularization timescale and the torus is being drained at a super-Eddington rate while forming.

Motivation & Objective

• To understand the dynamical evolution of stellar debris after tidal disruption in highly eccentric orbits.
• To resolve the discrepancy between analytical models and observed tidal disruption events by simulating debris evolution from first principles.
• To determine how cooling efficiency influences the final structure and accretion rate of the debris.

Proposed method

• Perform 3D hydrodynamical simulations of stars on highly eccentric orbits around supermassive black holes.
• Track the evolution of stellar debris following disruption, including shock heating and energy deposition from stream self-interaction.
• Model relativistic apsidal precession to capture the mechanism driving stream self-crossing and circularization.
• Vary cooling efficiency to compare outcomes with and without rapid energy radiative loss.
• Use viscous accretion models to assess accretion rates and structural evolution post-circularization.
• Analyze the resulting debris morphology and accretion dynamics under different physical conditions.

Experimental results

Research questions

• RQ1 How quickly does stellar debris circularize after tidal disruption, and what physical mechanism drives this process?
• RQ2 What determines whether the debris forms a narrow ring or an extended torus after circularization?
• RQ3 How do cooling timescales affect the accretion rate and structural stability of the debris?
• RQ4 What is the accretion rate of the debris during the circularization phase, and does it exceed the Eddington limit?
• RQ5 How does radiation pressure influence the evolution of the accreting debris when cooling is inefficient?

Key findings

• Stellar debris circularizes on an orbital timescale due to relativistic apsidal precession, which induces stream self-crossing.
• Higher eccentricity and deeper encounters accelerate the circularization process.
• Efficient cooling leads to the formation of a narrow, super-Eddington accreting ring at the circularization radius.
• Suppressed cooling results in an extended, centrifugally supported torus that forms while being drained at super-Eddington rates.
• The viscous timescale becomes comparable to the circularization timescale when cooling is inefficient, leading to concurrent torus formation and accretion.
• Radiation pressure causes puffing up of the accreting material when cooling is rapid, altering the accretion geometry.