Evolution of Accretion Disks with Mass Evaporation during the Quiescence of WZ Sge-Type Dwarf Novae and X-Ray Novae

Abstract

We propose a new scenario for disk evolution during the quiescence of WZ Sge-type dwarf novae and X-ray novae. A hole created by evaporation of the inner disk material steadily expands towards larger radii, eventually removing thermally unstable portions of the disk. The disk is, however, not in a steady state, since a certain amount of angular momentum is continuously brought in by an incoming stream, whereas angular-momentum losses due to the evaporation and to the tidal action by a companion star are likely to be small in the quiescence. Accordingly, the total angular momentum of the disk monotonically increases, whereas an increase in the disk mass is somewhat slower because of the mass evaporation. This results in an expansion of the disk and an accumulation of mass in the outermost part of the disk. The quiescence will be terminated if a thermal instability is ignited before the disk expands to the critical radius for the 3:1 resonance between Kepler and orbital angular frequencies. If not, superhumps are expected during quiescence.