A magnetic propeller in the cataclysmic variable AE Aquarii

Current treatments of accretion flows in magnetic cataclysmic variables (CVs) imply that the criterion for disc formation depends only on the accretion rate and the magnetic field strength, and is independent of the spin rate of the accreting star. The low accretion rate in the moderately magnetic CV AE Aqr should allow disc formation, yet the single-peaked Balmer emission lines show no evidence of a disc. We interpret these data as evidence that the rapidly rotating white dwarf in AE Aqr ejects most of the matter transferred from the secondary. The possibility of such states was pointed out in earlier theoretical work modelling the gas flow as inhomogeneous and diamagnetic. Explicit application of this model to AE Aqr accounts for the observed spin-down of the 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 than or similar to 10(17) g s(-1) respectively. We show that AE Aqr is likely to alternate between phases of disc accretion, in which the white dwarf spins up, and propeller states (as observed) in which it spins down. We comment on the likely consequences for other magnetic systems.