Hydrodynamic Simulation of Accretion Disks in Cataclysmic Variables

Abstract

The non-axisymmetric configurations of accretion disks in cataclysmic variables are studied by two-dimensional hydrodynamical calculations. The particle-hydrodynamic method first developed by Lin and Pringle (1976) is used, and time evolution of accretion disks under a constant mass supply rate from the secondary is followed until the system settles to a quasi-steady state. It is found that a binary system with comparable masses of component stars settles to a steady state of an elongated disk fixed in the rotating frame of the binary while in the case of low mass secondary it settles to a periodic oscillating state in which a non-axisymmetric (eccentric) structure is formed and it rotates slowly in the inertial frame of reference. The period of oscillation is a few percent longer than the orbital period of binary, and it offers a natural explanation to the “superhump” periodicity of SU UMa stars. Our results thus confirm basically those of Whitehurst (1988) who has discovered the tidal instability of an accretion disk in a low mass secondary.