Magnetic activity in accretion disc boundary layers

TL;DR

This study uses 3D magnetohydrodynamic simulations to analyze the boundary layer between an accretion disk and a star, revealing highly variable, magnetically amplified regions that could influence observable emissions and outflows.

Contribution

It provides new insights into the magnetic field amplification and structure of the boundary layer in accretion disks through detailed 3D simulations.

Findings

Magnetic fields are strongly amplified by shear in the boundary layer.

The transition region is narrow and highly variable.

Most energy dissipation occurs in high-density gas near the stellar envelope.

Abstract

We use three dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle (1989), the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission, and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.