HST GHRS Observations of a Silicon SPOT on the White Dwarf in V471 Tauri

Details HST GHRS Observations of a Silicon SPOT on the White Dwarf in V471 Tauri HST GHRS Observations of a Silicon SPOT on the White Dwarf in V471 Tauri

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.1503s&link_type=abstract

American Astronomical Society, 191st AAS Meeting, #15.03; Bulletin of the American Astronomical Society, Vol. 29, p.1232

Astronomy and Astrophysics

Astronomy

Scientific paper

We have used the Hubble Space Telescope Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, Si 3 lambda 1206, 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 Si 3 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 x 10(-18) M_sun 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 Si 3 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.

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