Time-dependent accretion onto magnetic white dwarfs - Effects of cyclotron emission

Previous accretion models have shown that if the cooling is due to optically thin bremsstrahlung, the flow is unstable. The shock height oscillates with a period of about 1 s, which corresponds to the cooling time for parameters characteristic of the AM Her binaries. It is shown that the effects of cyclotron cooling stabilize the flow if the magnetic field is large enough. For a 0.5 solar mass white dwarf accreting 1 g/sq cm sec, the flow is stable if B is more than or approximately equal to 2 x 10 to the 7th gauss. It is shown that the bremsstrahlung X-ray emission and the optical cyclotron emission are 180 deg out of phase when oscillations are present. The relevance of these calculations to the observed optical oscillations, with periods of about 1 s, in two AM Her stars is discussed.