Spreading of Accreted Material on White Dwarfs

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 corotation 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-ray, this material moves toward the pole as new material piles up behind it, eventually becoming part of the WD once it has a temperature and rotational velocity comparable to those at the surface. We present a set of solutions that describe the properties of this "spreading layer" in the steady state limit on the basis of the conservation equations recently derived by Inogamov and Sunyaev 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 = 0.6 M ☉ and radius R = 9 × 10 8 cm. These solutions show that the spreading layer typically extends to an angle of θ SL ≈ 0.01-0.1 (with respect to the equator), depending on accretion rate and the magnitude of the viscosity. At low accretion rates, ≲ 10 18 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, the material may spread to latitudes high enough to be visible above the accretion disk. The effective temperature of the spreading layer is ~(2-5) × 10 5 K with approximately T eff ∝ 9/80 . This power-law dependence on is weaker than for a fixed radiating area and may help explain extreme-ultraviolet observations during dwarf novae. We speculate about other high accretion rate systems ( ≳ 10 18 g s -1 ) that may show evidence for a spreading layer, including symbiotic binaries and supersoft sources.