Simulations of the Boundary Layer Between a White Dwarf and its Accretion Disk

TL;DR

This study uses 2.5D simulations to explore the structure and dynamics of the boundary layer between a white dwarf and its accretion disk, revealing effects of viscosity on wave formation and optical thickness.

Contribution

It introduces a numerical approach to model the boundary layer in non-magnetic cataclysmic variables, highlighting the impact of viscosity on wave development and optical properties.

Findings

High viscosity (alpha=0.1) causes gravity waves in the boundary layer.

Boundary layer extends over 30 degrees from the disk plane.

Low viscosity (alpha=0.001) results in no gravity waves and an optically thin boundary layer.

Abstract

Using a 2.5D time-dependent axisymmetric numerical code we recently developed, we solve the full compressible Navier-Stokes equations (including an alpha-viscosity prescription) to determine the structure of the boundary layer between the white dwarf and the accretion disk in non-magnetic cataclysmic varia ble systems. In this preliminary work, our numerical approach does not include radiation. In the energy equation, we either take the dissipation function (Phi) into account or we assumed that the energy is instantly radiated away (Phi). For a slowly rotating non magnetized accreting white dwarf, the accretion disk e xtends all the way to the stellar surface. There, the matter impacts and spread s towards the poles as new matter continuously piles up behind it. We carried out numerical simulations for different values of the alpha viscosity parameter (alpha), corresponding to different mass accretion rates. In the high viscosity cases (alpha=0.1), the spreading boundary layer sets off a gravity wave in the s urface matter. The accretion flow moves supersonically over the cusp making it s usceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz shea ring instabilities. This BL is optically thick and extends more than 30 degrees to either side of the disk plane after only 3/4 of a Keplerian rotation period (19s). In the low viscosity cases (alpha=0.001), the spreading boundary layer does not set off gravity waves and it is optically thin.