Magnetism in Binary Stars
Magnetic fields are of fundamental importance in close binary stars due to the angular momentum transport that can occur via the action of magnetic stresses. The strongly magnetic AM Herculis stars and the intermediate polars are believed to account for approximately 25% of all cataclysmic variables. In the AM Her binaries an accretion disc cannot form and the accretion stream is magnetically channelled to form at least one localized column above the white dwarf surface. The strongly magnetic primary star usually spins in synchronism with the orbit, even though it will experience a strong magnetically influenced accretion torque. Partially disrupted discs form in the intermediate polars and an inner accretion curtain flow channels matter on to the primary. The X-ray binary pulsars and the accreting millisecond pulsars have magnetic neutron stars which partially disrupt their accretion discs.
Sub-thermal magnetic fields lead to a magnetorotational instability in accretion discs which is believed to be the source of the turbulent viscosity needed to account for the inflow. Disc dynamos can generate large-scale magnetic fields which can lead to radial transport of angular momentum, or its vertical removal via a channelled wind flow, depending on the magnetic field geometry. Dynamos can generate magnetic fields in secondary stars, and channelled wind flows lead to a braking torque. This, together with tidal coupling, can cause orbital angular momentum loss that is consistent with the mass transfer rates believed to occur in cataclysmic variables above the period gap. These topics are outlined here.