The vertical structure of the boundary layer around compact objects

Abstract

Context. Mass transfer due to Roche lobe overflow leads to the formation of an accretion disk around a weakly magnetized white dwarf (WD) in cataclysmic variables. At the inner edge of the disk, the gas comes upon the surface of the WD and has to get rid of its excess kinetic energy in order to settle down on the more slowly rotating outer stellar layers. This region is known as the boundary layer (BL). Aims. In this work we investigate the vertical structure of the BL, which is still poorly understood. We shall provide details of the basic structure of the two-dimensional (2D) BL and how it depends on parameters such as stellar mass and rotation rate, as well as the mass-accretion rate. We further investigate the destination of the disk material and compare our results with previous one-dimensional (1D) simulations. Methods. We solve the 2D equations of radiation hydrodynamics in a spherical geometry using a parallel grid-based code that employs a Riemann solver. The radiation energy is considered in the two-temperature approach with a radiative flux given by the flux-limited diffusion approximation. Results. The BL around a non-rotating WD is characterized by a steep drop in angular velocity over a width of only 1% of the stellar radius, a heavy depletion of mass, and a high temperature (500 000 K) as a consequence of the strong shear. Variations in Om_star, M_star, and M_dot influence the extent of the changes of the variables in the BL but not the general structure. Depending on Om_star, the disk material travels up to the poles or is halted at a certain latitude. The extent of mixing with the stellar material also depends on Om_star. We find that the 1D approximation matches the 2D data well, apart from an underestimated temperature.