Magnetic Discs

Abstract

We consider accretion flows on to rotating magnetic white dwarfs. For realistic parameters the usual simple criteria for disc formation and disruption involving comparison of magnetic and material stresses are not relevant, as the field takes several dynamical times to penetrate the matter. Instead the flow consists of diamagnetic blobs interacting with the field through a surface drag term. A disc forms in the usual way only if the viscous time near the circularization radius is shorter than the magnetic drag timescale there. This criterion places rather lower limits (~ 10⁴ G) on the white dwarf fieldstrengths allowing simple disc formation than hitherto, a result in agreement with the short spin periods of systems such as DQ Her. Flows on to stronger fields are also disc-like, in the sense that matter outside the corotation radius is closely confined to the orbital plane and spreads out into a disc shape. The flow pattern is rather different from a standard Keplerian accretion disc however. This type of flow can be shown analytically to produce the correct equilibrium spin behaviour for intermediate polars. We show that the mass transfer from the companion is likely to be dynamically unstable except very near spin equilibrium of the white dwarf, so that deviations from equilibrium should be short-lived. Simple estimates show that in equilibrium the companion rė-accretes rat her more of the mass it loses through L₁ than is captured by the white dwarf. Numerical studies show that the velocity field of the flow is very sensitive to the instantaneous overflow rate