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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
Exhibit Hall,
We have used the Hubble Space Telescope Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, \ionSi3 \lambda1206, in the ultraviolet spectrum of the magnetic white dwarf component of the Hyades pre-cataclysmic binary V471 Tauri (Sion et al. 1997, ApJL, submitted). The \ionSi3 feature is modulated on the X-ray/EUV/optical 9.25-minute rotational period of the white dwarf, and is strongest at the time of X-ray/EUV minimum and optical maximum. A model in which the X-ray/EUV magnetic pole is dark due to metallic absorption, and bright in the optical due to flux redistribution, is strongly supported. We derive a Si abundance of 0.1 times the solar value in the accretion cap, which is assumed to cover 40% of the visible hemisphere of the white dwarf. Assuming equilibrium between mass accretion onto, and diffusion of Si out of, the photosphere allows an estimate of the accretion rate onto the white dwarf, which we find to be accreting from the dwarf K star companion's wind at the extremely low rate of 3 \times 10^-18 M_ødot yr^-1. This is some five orders of magnitude lower than the Bondi-Hoyle fluid rate and strongly suggests operation of a magnetic-centrifugal propeller mechanism which rejects most of the material that attempts to accrete. The \ionSi3 feature may also be split into two Zeeman-shifted \sigma components, while the \pi component is not seen. If real, the apparent magnitude of the splitting and the lack of the \pi component are consistent with a field strength near 200 kilogauss and a viewing geometry in which the magnetic field of the accreting spot is seen pole-on.
