The Effects of Cooling on Boundary Layer Accretion

TL;DR

This paper explores how different cooling rates affect wave dynamics and mass transport in boundary layer accretion systems, revealing that intermediate cooling can significantly suppress transport and influence variability.

Contribution

It provides a detailed analysis of the impact of cooling timescales on wave behavior and transport in boundary layer accretion, highlighting the importance of thermodynamics in these regions.

Findings

Fast or slow cooling minimally affects wave dynamics.

Intermediate cooling rates can suppress mass and angular momentum transport.

Cooling influences wave wavenumbers and may cause low-frequency variability.

Abstract

In many cases accretion proceeds from disks onto planets, stars, white dwarfs, and neutron stars via a boundary layer, a region of intense shear where gas transitions from a near-Keplerian speed to that of the surface. These regions are \textit{not} susceptible to the common magnetorotational and Kelvin-Helmholtz instabilities, and instead global modes generated by supersonic shear instabilities are a leading candidate to govern transport in these regions. This work investigates the dynamics of these systems under a range of thermodynamic conditions, surveying both disk sound speeds and cooling rates. Very fast or very slow cooling has little effect on wave dynamics: in the fast-cooling limit, waves propagate in an effectively isothermal manner, and in the slow limit wave propagation is effectively adiabatic. However, when the cooling timescale is comparable to the wave period, wave damping becomes extreme. In cases with intermediate cooling rates, mass and angular momentum transport can be suppressed by orders of magnitude compared to isothermal and uncooled cases. Cooling in accretion disks leads to a preference for wavenumbers near and below the Mach number of the disk; the corresponding lower frequencies can (in non-isothermal systems) couple to gravity modes within the star, potentially causing low-frequency variability such as dwarf nova and quasi-periodic oscillations in accreting systems.