Hubble Space Telescope observations of an accreted silicon spot on the white dwarf in V471 Tauri

We have used the Hubble Space Telescope's (HST) Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, Si III lambda 1206, in the spectrum of the magnetic white dwarf component of the Hyades precataclysmic binary V471 Tauri. The Si III feature is modulated on the soft X-ray/EUV/optical 9.25 minute rotational period of the white dwarf and is strongest at the time of soft X-ray/EUV minimum and optical maximum. A model in which the soft X-ray/EUV magnetic pole is dark because of metallic and helium absorption, and bright in the optical due to flux redistribution, is strongly supported, We derive a Si abundance of 0.10(-0.07)(+0.03) times solar in the accretion cap. Assuming equilibrium between mass accretion onto-and diffusion of Si out of-the photosphere, we find the white dwarf to be accreting from its dK companions wind at only 3.8 x 10(-18) M(circle dot) yr(-1), some 5 orders of magnitude lower than the Bondi-Hoyle fluid rate. This strongly suggests operation of a magnetic-centrifugal propeller mechanism that rejects most of the material that attempts to accrete. We tentatively detect Zeeman splitting of the Si III line, implying a polar field strength of similar to 350 kG, V471 Tau is destined, in the distant future, to become a DQ Her-type cataclysmic binary.