AM Her Stars: Stream Channelling and the Accretion Torque

Abstract

The strong magnetic moment of the primary star results in a surrounding magnetic field which completely prevents the formation of an accretion disc. Matter lost from the L ₁ region of the secondary becomes channelled by the primary’s magnetic field and hence, in general, forms a 3D accretion flow. The flow causes distortions of the stellar magnetic field and the resulting stresses act through the stream to transfer angular momentum to the star. The consequent accretion torque depends on the orientation of the primary’s magnetic axis and has components parallel and normal to the orbital angular momentum vector. These components can have comparable magnitudes for a significant range of magnetic orientations. The synchronous and asynchronous accretion torques are considered here, as well as the effects of partial field channelling. For a dipolar magnetic field and moderate degrees of asynchronism, the normal components of the accretion torque average to zero over a synodic rotation period, while the parallel component has a conservative averaged form for total field channelling.