Angular Momentum Transport by Acoustic Modes Generated in the Boundary Layer I: Hydrodynamical Theory and Simulations

TL;DR

This paper develops a hydrodynamical theory supported by simulations showing that acoustic waves excited by shear in accretion disk boundary layers efficiently transport angular momentum nonlocally, challenging traditional turbulent stress models.

Contribution

It introduces a new theoretical framework for angular momentum transport via acoustic modes in boundary layers, supported by 3D hydrodynamical simulations, identifying three global wave modes and their dispersion relations.

Findings

Identification of three types of global acoustic modes.

Angular momentum transport is nonlocal and driven by wave radiation.

Transport mechanism differs from traditional turbulent stress models.

Abstract

The nature of angular momentum transport in the boundary layers of accretion disks has been one of the central and long-standing issues of accretion disk theory. In this work we demonstrate that acoustic waves excited by supersonic shear in the boundary layer serve as an efficient mechanism of mass, momentum and energy transport at the interface between the disk and the accreting object. We develop the theory of angular momentum transport by acoustic modes in the boundary layer, and support our findings with 3D hydrodynamical simulations, using an isothermal equation of state. Our first major result is the identification of three types of global modes in the boundary layer. We derive dispersion relations for each of these modes that accurately capture the pattern speeds observed in simulations to within a few percent. Second, we show that angular momentum transport in the boundary layer is intrinsically nonlocal, and is driven by radiation of angular momentum away from the boundary layer into both the star and the disk. The picture of angular momentum transport in the boundary layer by waves that can travel large distances before dissipating and redistributing angular momentum and energy to the disk and star is incompatible with the conventional notion of local transport by turbulent stresses. Our results have important implications for semianalytical models that describe the spectral emission from boundary layers.