RADIATION-DRIVEN DISKONS - AN OVERVIEW

The current status of the theory of a new astrophysical phenomenon, a radiation-driven diskon, is outlined. The cyclotron radiation pressure around sufficiently hot, strongly magnetized white dwarfs and neutron stars is shown to be able to drive a wind from the photosphere and su pport a plasma envelope in the closed part of the magnetosphere. The m agnetohydrostatic configuration of an optically thin, radiatively supp orted plasma envelope is determined. It consists of an equatorial disk in the region where the cyclotron radiation force exceeds the local f orce of gravity and a closed shell near the equilibrium surface where the radiation pressure equals gravity. The effects of finite optical d epth on the behaviour of the magnetospheric plasma and the influence o f the envelope on the observed radiation are discussed. Classes of mag netic degenerate stars are pointed out in which radiation-driven disko ns may be found. The best candidates are two individual stars, the str ongly magnetized white dwarfs GD 229 and PG 1031+234. Both exhibit bro ad and deep depressions in the ultraviolet which are explained as a re sult of cyclotron scattering by an optically thick radiation-driven en velope in the inhomogeneous magnetic field of the star. We predict a t emporal and spectral variability of these features due to non-stationa ry plasma motions in the envelope.