Astronomy & Astrophysics Seminar: Tidal disruption of a magnetized star

Zoom: https://tau-ac-il.zoom.us/j/87185167411?pwd=FpTUyIgXaOturzOFSetA6pPVYQilen.1

Abstract:

Tidal disruptions of stars by supermassive black holes in galactic centres are now being actively studied both theoretically and observationally. Such tidal disruption events (TDEs) are observed throughout the electromagnetic spectrum, from radio to gamma-rays. It is still unclear how the emission is produced and, in particular, what is the role of the magnetic field of the disrupted star. There are many ways how magnetic fields might affect the dynamics of a TDE. For example, they are likely responsible for angular momentum transfer in the accretion disc formed at later stages and thus affect the X-ray and UV radiation associated with the disc. Magnetic fields are also an important requirement for the formation of relativistic jets, that are seen in a small minority of TDEs. We report the first numerical simulation of a TDE spanning the whole disruption process, from a star on a parabolic orbit towards the formation of a circularized disc.

Most of the magnetic field amplification is related to the nozzle shock that forms when stellar debris returns to the pericentre of the orbit in the form of a fallback stream. The fields in the fallback stream entering the shock are aligned with the flow lines and are similar in strength to the initial field within the star. Downstream of the shock, the flow is turbulent, and the magnetic field becomes chaotic with a typical correlation length of the order the width of the fallback stream. During the circularization, magnetic fields evolve toward equipartition with the toroidal fields slightly stronger than poloidal.
 

Seminar Organizer: Dr. Jonathan Stern