Angular Momentum Transport and Variability in Boundary Layers of Accretion Disks Driven by Global Acoustic Modes

TL;DR

This paper uses high-resolution hydrodynamical simulations to reveal how global acoustic modes in boundary layers of accretion disks facilitate angular momentum transport and may explain observed variability in accreting compact objects.

Contribution

It demonstrates the existence of stable, global acoustic modes in boundary layers that drive angular momentum transport without magnetic fields, a novel insight into disk dynamics.

Findings

Global acoustic modes are trapped and stable for hundreds of orbits.

Dissipation of modes in weak shocks transports angular momentum.

Modes potentially explain variability in accreting compact objects.

Abstract

Disk accretion onto a weakly magnetized central object, e.g. a star, is inevitably accompanied by the formation of a boundary layer near the surface, in which matter slows down from the highly supersonic orbital velocity of the disk to the rotational velocity of the star. We perform high resolution 2D hydrodynamical simulations in the equatorial plane of an astrophysical boundary layer with the goal of exploring the dynamics of non-axisymmetric structures that form there. We generically find that the supersonic shear in the boundary layer excites non-axisymmetric quasi-stationary acoustic modes that are trapped between the surface of the star and a Lindblad resonance in the disk. These modes rotate in a prograde fashion, are stable for hundreds of orbital periods, and have a pattern speed that is less than and of order the rotational velocity at the inner edge of the disk. The origin of these intrinsically global modes is intimately related to the operation of a corotation amplifier in the system. Dissipation of acoustic modes in weak shocks provides a universal mechanism for angular momentum and mass transport even in purely hydrodynamic (i.e. non-magnetized) boundary layers. We discuss the possible implications of these trapped modes for explaining the variability seen in accreting compact objects.