A MAGNETIC PROPELLER IN THE CATACLYSMIC VARIABLE AE AQUARII

Current treatments of accretion flows in magnetic cataclysmic variable s (CVs) imply that the criterion for disc formation depends only on th e accretion rate and the magnetic field strength, and is independent o f the spin rate of the accreting star. The low accretion rate in the m oderately magnetic CV AE Aqr should allow disc formation, yet the sing le-peaked Balmer emission lines show no evidence of a disc. We interpr et these data as evidence that the rapidly rotating white dwarf in AE Aqr ejects most of the matter transferred from the secondary. The poss ibility of such states was pointed out in earlier theoretical work mod elling the gas flow as inhomogeneous and diamagnetic. Explicit applica tion of this model to AE Aqr accounts for the observed spin-down of th e white dwarf, and gives estimates of the white dwarf magnetic moment and the mass transfer rate as similar to 10(32) G cm(3) and greater th an or similar to 10(17) g s(-1) respectively. We show that AE Aqr is l ikely to alternate between phases of disc accretion, in which the whit e dwarf spins up, and propeller states (as observed) in which it spins down. We comment on the likely consequences for other magnetic system s.