Boundary Layers of Accretion Disks: Wave-Driven Transport and Disk Evolution

TL;DR

This study investigates how acoustic wave modes in the boundary layer of accretion disks facilitate angular momentum and mass transport, influencing disk evolution, through hydrodynamic simulations and analysis.

Contribution

It provides a comparative analysis of wave mode transport properties and identifies the modes responsible for mass accretion, advancing understanding of wave-mediated disk evolution.

Findings

Wave modes significantly contribute to angular momentum transport.

Correlated surface density and radial velocity perturbations drive mass accretion.

Transport efficiency weakly depends on Mach number.

Abstract

Astrophysical objects possessing a material surface (white dwarfs, young stars, etc.) may accrete gas from the disc through the so-called surface boundary layer (BL), in which the angular velocity of the accreting gas experiences a sharp drop. Acoustic waves excited by the supersonic shear in the BL play an important role in mediating the angular momentum and mass transport through that region. Here we examine the characteristics of the angular momentum transport produced by the different types of wave modes emerging in the inner disc, using the results of a large suite of hydrodynamic simulations of the BLs. We provide a comparative analysis of the transport properties of different modes across the range of relevant disc parameters. In particular, we identify the types of modes which are responsible for the mass accretion onto the central object. We find the correlated perturbations of surface density and radial velocity to provide an important contribution to the mass accretion rate. Although the wave-driven transport is intrinsically non-local, we do observe a clear correlation between the angular momentum flux injected into the disc by the waves and the mass accretion rate through the BL. We find the efficiency of angular momentum transport (normalized by thermal pressure) to be a weak function of the flow Mach number. We also quantify the wave-driven evolution of the inner disc, in particular the modification of the angular frequency profile in the disc. Our results pave the way for understanding wave-mediated transport in future three-dimensional, magnetohydrodynamic studies of the BLs.