Cataclysmic Variables as a Magnetic Laboratory

Abstract

Cataclysmic variables are very close binary stars in which the primary component is a white dwarf. The secondary star, usually a low-mass main-sequence star, is filling its Roche lobe and transferring material to the white dwarf. This material has too much angular momentum to fall directly on to the white dwarf and instead forms an accretion disc, which makes up the third major component of the cataclysmic variable. A magnetic field can be associated with each of the two stars and the disc. In the case of the white dwarf the field is frozen in. It can be strong enough to disrupt all or part of the accretion disc and force the accreting matter to fall in along field lines. The fields associated with the red dwarf and accretion disc appear to be dynamo generated. They cannot be measured directly but must be inferred from other properties of the systems. In cataclysmic variables, conservative mass transfer would lead to the secondary star shrinking within its Roche lobe until it expanded on a nuclear timescale. For almost all systems this exceeds the age of the Galaxy by several orders of magnitude. Nor can simple mass loss drive the observed mass transfer and an alternative angular momentum loss mechanism is required. At the shortest periods gravitational radiation suffices but at longer periods magnetic braking seems to be more likely. Any determination of the mass transfer rate can then constrain the field strength and structure and ultimately the nature of any magnetic dynamo. In the absence of viscosity, transferred material would accumulate in a ring around the white dwarf. Viscosity allows this ring to spread out into a disc so that matter accretes while angular momentum is returned to the orbit. Some form of radial magnetic field in the disc provides the most viable source of viscosity. It appears to be necessary to generate this field by dynamo action within the disc itself. Dwarf novae undergo periodic outbursts that are due to changes in accretion rate through the disc. This time dependent behaviour probes the strength of the radial and azimuthal field and hence any disc dynamo.