Stellar Magnetic Fields

Abstract

The origins of the stellar magnetic fields in accreting binary systems are considered. Observations and theoretical models suggest that the M and K dwarf secondary stars should have dynamo generated magnetic fields, with surface values up to several kG, including large-scale structures. Significant magnetic fields can be generated in the fully convective secondaries, corresponding to the lower mass M dwarfs, even though these stars do not have a tachocline region. Rapid rotation of the tidally synchronized secondary together with strong convective motions leads to effective α ² dynamo action. The higher mass M dwarfs and the K dwarfs have radiative cores and hence will possess an over-shoot tachocline region connecting the core to the convective envelope. This may affect the nature of the dynamo, particularly if some differential rotation remains across this transition layer.

The magnetic fields of the white dwarf and neutron star primaries are believed to be of fossil origin, but the binary formation process may have played a major role in their determination. Also, accretion of material can cause the degenerate primary to contract as it gains mass. This can lead to significant modification of the surface field due to advection, within the lifetime of the system, depending on the efficiency of magnetic buoyancy.