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A B S T R A C TThe stand-off shock formed in the accretion flow on to a stationary wall, such as the surface of a white dwarf, may be thermally unstable, depending on the cooling processes which dominate the post-shock flow. Some processes lead to instability, while others tend to stabilize the shock. We consider competition between the destabilizing influence of thermal bremsstrahlung cooling, and a stabilizing process which is a power law in density and temperature. Cyclotron cooling and processes which are of order 1, 3/2 and 2 in density are considered. The relative efficiency and power-law indices of the second mechanism are varied, and particular effects on the stability properties and frequencies of oscillation modes are examined.

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When particle methods are used to simulate the high Mach number collision of gas clouds particle streaming may occur because the particles can penetrate the interface between the clouds. We show how this penetration can be prevented by using an appropriate artificial viscosity. Numerical experiments in one and three dimensions are described.

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Stability of accretion onto magnetic white dwarfs is investigated via perturbation analysis. We consider a cooling function with composite power laws, and the power-law indices are appropriate for bremsstrahlung and optically thick cyclotron cooling. We find that the presence of cyclotron cooling generally suppresses the instability, consistent with numerical simulations by other investigators.