Viscosity prescriptions in accretion discs with shock waves

Abstract

We study the evolution of viscous, isothermal, rotating, thin, axisymmetric accretion discs around compact objects using smoothed particle hydrodynamics. We emphasize the effects of different choices of viscosity prescription on the evolution of angular momentum, as well as on other physical properties of the disc. We show that a flow with the Shakura-Sunyaev viscosity prescription may produce a mixed shock, where the angular momentum can also change significantly across the shock. We present and study the effects of other prescriptions of viscosity which render the angular momentum continuous across the shock waves. In general, it is observed that, for flows with a small viscosity, the shocks are weaker, form farther away and are wider as compared with the shocks in inviscid flows. If the viscosity is high, shocks do not form at all. The flow remains subsonic and Keplerian throughout the disc and becomes supersonic only very close to the horizon.