MAGNETIC BRAKING IN MAGNETIC BINARY STARS

The role of an external magnetic field in the magnetic braking of a st ar with a dipolar field is investigated. In a magnetic cataclysmic var iable system (i.e. the primary compact star has a strong magnetic fiel d), the field external to the braking star (a late-type main-sequence star with a dynamo-generated field) originates from the compact star. A closed field region - the system dead zone - is formed within the bi nary system, and it does not take part in magnetic braking. The overal l braking rate depends on the extent of this region and of the open fl ux, and is dependent on centrifugal effects. In the case of two intera cting dipoles, the dipole orientations relative to the spin axes and t o each other are found to be important, leading to different amounts o f open flux and therefore of magnetic braking, owing to different cent rifugal effects on closed field regions. However, in circumstances con sistent with observations and dynamo theory, the white dwarf's field r educes the magnetic braking of the secondary significantly, a finding qualitatively similar to the results previously obtained for two anti- aligned dipoles perpendicular to the orbital plane. In the cases where the two dipole axes are not perpendicular to the orbital plane, but a re inclined in the plane that links them, the 'cut-off' in magnetic br aking is less abrupt, and this effect is more obvious as the inclinati ons increase. Only in the extreme cases when the two dipole axes are a ligned in the orbital plane does the braking increase with white dwarf field strength, We conclude that detailed evolutionary modelling of A M Herculis systems needs to take account of the inclination effect.