Hydrodynamic simulations of cataclysmic variable accretion discs in outburst

Abstract

We study the outburst phase of cataclysmic variable (CV) accretion discs in the (r, π) plane with full hydrodynamics. Vertically the disc is treated using the one-zone model of Stehle & Spruit, which allows us to follow the fundamental mode of vertical disc oscillations correctly. α-type shear viscosity (Shakura & Sunyaev) and energy loss at the surface of the accretion disc by radiation are included. We run accretion disc models with different masses M1 and M2 for the primary and the secondary stars and different values of α. In all calculations we observe strong spiral shock arms, which dominate the disc evolution on a hydrodynamical time-scale. In cases where the surface mass density Σ (r) decreases with radius r the disc pattern is stable. In cases where Σ (r) increases with r we observe a quasi-periodic cycle of disc size. During the phase where the disc is large, the spiral arm close to the L1 point connects to the secondary, and about 1 per cent of the total disc mass is lost to the secondary per cycle. However, in none of our calculations are we able to find an eccentric and precessing accretion disc, even though the disc edge is outside the 3:1 tidal resonance radius R31. Our calculations therefore cannot confirm the tidal instability model, presently the only model for superhumps in SU UMa CVs during superoutbursts.