?Swinging dipoles? inmagnetic close binary stars

Abstract

The recently discovered phenomena in polars — Magnetic Close Binary Stars (MCBS), are discussed. Asynchronous MCBS are on the ‘propeller stage’ (which is analogous to the investigated one for neutron stars), and are synchronizing during tS≲103 yr, being during this stage rapidly evolving, mass ejecting systems. The accretion rate is largest when the angle θ between the magnetic axis and the line of centers is near zero. If θ≈90°, then the magnetic field prevents the plasma flow, and the ‘magnetic valve’ becomes closed. Near this position the oscillations of the orientation of the white dwarf's magnetic axis may be excited. This model of a ‘swinging dipole’ has such observational properties: (a) the system's luminosity changes with characteristical times of some years; (b) the phase curves of light, polarization and radial velocities must be cyclically changing with a few year cycle; (c) one would observe the correlation between the phase shift of these curves, and the system's luminosity. The observational data on polars are briefly discussed. All these phenomena are observed in AM Herculis and some other polars, but subsequent studies are needed to investigate the orientation changes of the magnetic axis, and so the structure and evolution of this exotic class of our Galaxy population.