Spreading of Accreted Material on White Dwarfs

Abstract

When a white dwarf (WD) is weakly magnetized and its accretion disk is thin, accreted material first reaches the WD's surface at its equator. This matter slows its orbit as it comes into co-rotation with the WD, dissipating kinetic energy into thermal energy and creating a hot band of freshly accreted material around the equator. Radiating in the extreme ultraviolet and soft X-rays, this material moves toward the pole as new material piles behind it, eventually becoming part of the WD once it has a temperature and rotational velocity comparable with the surface. We present a set of solutions which describe the properties of this ``spreading layer'' in the steady state limit based on the conservation equations derived by Inogamov & Sunyaev (1999) for accreting neutron stars. Our analysis and subsequent solutions show that the case of WDs is qualitatively different. We investigate example solutions of the spreading layer for a WD of mass $M=0.6M_\odot$ and radius $R=9\times10^{8}{\rm cm}$. These solutions show that the spreading layer typically extends to an angle of $\theta_{\rm SL}\approx0.01-0.1$ (with respect to the equator), depending on accretion rate and the magnitude of the viscosity. At low accretion rates, $\dot{M}\lesssim10^{18}{\rm g s}^{-1}$, the amount of spreading is negligible and most of the dissipated energy is radiated back into the accretion disk. When the accretion rate is high, such as in dwarf novae, symbiotic binaries, and supersoft sources, the material may spread to latitudes high enough to be directly visible above the accretion disk.