Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer II: MHD Simulations

TL;DR

This study uses 3D MHD simulations to show that acoustic waves are a significant mechanism for angular momentum transport in the boundary layer between accretion disks and stars, challenging the traditional turbulent viscosity model.

Contribution

It demonstrates that wave-driven angular momentum transport in the boundary layer is dominant over MRI and suggests a global wave dissipation process rather than local turbulence.

Findings

Waves can dominate angular momentum transport near the boundary layer.

Magnetic fields are amplified but remain subthermal.

Wave dissipation and shocks are key to angular momentum transfer.

Abstract

We perform global unstratified 3D magnetohydrodynamic simulations of an astrophysical boundary layer (BL) -- an interface region between an accretion disk and a weakly magnetized accreting object such as a white dwarf -- with the goal of understanding the effects of magnetic field on the BL. We use cylindrical coordinates with an isothermal equation of state and investigate a number of initial field geometries including toroidal, vertical, and vertical with zero net flux. Our initial setup consists of a Keplerian disk attached to a non-rotating star. In a previous work, we found that in hydrodynamical simulations, sound waves excited by shear in the BL were able to efficiently transport angular momentum and drive mass accretion onto the star. Here we confirm that in MHD simulations, waves serve as an efficient means of angular momentum transport in the vicinity of the BL, despite the magnetorotational instability (MRI) operating in the disk. In particular, the angular momentum current due to waves is at times larger than the angular momentum current due to MRI. Our results suggest that angular momentum transport in the BL and its vicinity is a global phenomenon occurring through dissipation of waves and shocks. This point of view is quite different from the standard picture of transport by a local anomalous turbulent viscosity. In addition to angular momentum transport, we also study magnetic field amplification within the BL. We find that the field is indeed amplified in the BL, but only by a factor of a few and remains subthermal.