RADIATION-DRIVEN ENVELOPES AROUND MAGNETIC WHITE-DWARFS - RADIATION-DRIVEN DISKONS

We investigate the formation of a plasma envelope in the magnetosphere of a hot white dwarf by cyclotron radiation pressure. The radiation p ressure distribution (both in the continuum and in the cyclotron line) is derived for an optically thin, dipolar magnetosphere of an isolate d, non-rotating star emitting blackbody radiation. For an isothermal, fully ionized, pure hydrogen plasma the hydrostatic solution consists of a closed plasma shell accumulated in a potential well near the equi librium surface, where radiation pressure cancels gravity, and an equa torial disk inside this surface. The presence of a finite optical dept h leads to a temporal variation of the plasma envelope and of the obse rved radiation. We apply these results to the strongly magnetized whit e dwarf GD 229, which is a candidate for such a radiation-driven envel ope or ''diskon''. The deep unidentified 2000-3000 Angstrom depression in its UV spectrum is explained as the result of cyclotron scattering in optically thick gyroresonant layers around the star. We predict a temporal and spectral variability of this feature with a characteristi c time of greater than or equal to 1 hr due to non-stationary plasma m otions in the envelope.