Simulations of Magnetic Fields in Tidally-Disrupted Stars

TL;DR

This study uses magnetohydrodynamical simulations to explore how magnetic fields in stars are affected during tidal disruptions by supermassive black holes, revealing amplification and magnetic pressure evolution.

Contribution

First magnetohydrodynamical simulations of stellar tidal disruptions considering different magnetic field configurations and disruption outcomes.

Findings

Magnetic fields amplify up to twenty times in surviving stars.

No evidence of a self-sustaining dynamo during disruption.

Magnetic pressure in debris streams reaches equipartition with gas.

Abstract

We perform the first magnetohydrodynamical simulations of tidal disruptions of stars by supermassive black holes. We consider stars with both tangled and ordered magnetic fields, for both grazing and deeply disruptive encounters. When the star survives disruption, we find its magnetic field amplifies by a factor of up to twenty, but see no evidence for the a self-sustaining dynamo that would yield arbitrary field growth. For stars that do not survive, and within the tidal debris streams produced in partial disruptions, we find that the component of the magnetic field parallel to the direction of stretching along the debris stream only decreases slightly with time, eventually resulting in a stream where the magnetic pressure is in equipartition with the gas. Our results suggest that the returning gas in most (if not all) stellar tidal disruptions is already highly magnetized by the time it returns to the black hole.