The Spreading Layer and Dwarf Nova Oscillations

We describe recent theoretical work on the final stage of accretion when material passes from an accretion disk onto a white dwarf surface. Our calculations focus on understanding the latitudinal spreading and differentially rotating profile of this material, so we call it the “spreading layer” (SL) model. The SL typically extends to an angle of θSL ≈ 0.01−0.1 radians, with respect to the equator. At low accretion rates (Ṁ ∼< 10 18 g s) the amount of spreading is small, so that the dissipated energy is radiated back into the accretion disk. When Ṁ is high, such as in dwarf novae, symbiotic binaries, and supersoft sources, the material spreads to higher latitudes to be more easily observed. The SL may contain shallow surface modes, and we propose that such waves could produce dwarf nova oscillations (DNOs). This hypothesis naturally explains many key properties of DNOs, including their frequency range, sinusoidal nature, sensitivity to Ṁ , and why they are only seen during outburst.