Disk Structure in U Geminorum

Abstract

A hot spot at the outer rim of the accretion disk dominates the light of U Geminorum at minimum light. We take this as evidence that there is no accretion from the disk onto the white dwarf between the eruptions, and we assume there is no viscosity in the disk at that time. The hot spot is produced by dissipation of the radial component of velocity of stream falling from the inner Lagrangian point. Angular momentum per unit massis smaller in the stream than it is in the outer parts of the disk. This leads to angular momentum redistribution in the outer part of the disk. The difference of tangential velocity between the stream and the disk is dissipated in few orbital periods. These processes make the outer parts of the disk look like a torus. We calculated the structure of the torus in U Geminorum between the eruptions and we obtained the following oarameters: mass of the torus: 10^-9 - 10^-8 M_⊙ (assumed), its optical thickness: 10⁶, the large radius (i.e. the radius of the disk): 0.5 R_⊙ (assumed), the small radius (i.e. the half thickness of the outer parts of the disk): 0.05 R_⊙. Conditions at the surface of the torus are similar as on the solar surface.