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Astron. Astrophys. 361, 629-640 (2000)


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The nature of the DAB white dwarf HS 0209+0832 *

B. Wolff 1, S. Jordan 1, D. Koester 1 and D. Reimers 2

1 Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany (wolff,jordan,koester@astrophysik.uni-kiel.de)
2 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received 17 April 2000 / Accepted 11 July 2000

Abstract

The DAB white dwarf HS 0209+0832 is one of the few white dwarfs with detected helium in the DB gap between 28 000 and 45 000 K. We have obtained ultraviolet and optical spectra with the Hubble Space Telescope (HST). The analysis results in an effective temperature of about 35 000 K and a helium abundance of about 1%. The presence of C IV at 1548 Å, which was earlier detected by the International Ultraviolet Explorer (IUE), is confirmed. Additionally, we could identify more than 140 photospheric and 12 interstellar metal lines. For the first time we have found aluminum, calcium, titanium, nickel, and zinc, which cannot be supported by radiative levitation at such a low effective temperature. With stratified model atmospheres we tested, whether HS 0209+0832 has a very thin layer of hydrogen ([FORMULA] [FORMULA]) on top of the helium envelope so that it is not thick enough to hide all the helium as is usually assumed to explain the DB gap. From the detailed shape of the He II 1640 Å line we could exclude such a model and conclude that diffusion equilibrium has not yet been reached; all observations are compatible with almost homogeneously mixed atmospheres, as would also be expected from the presence of metal lines. The most probable explanation is that HS 0209+0832 is still accreting matter from an interstellar cloud and that helium and metals would sink downwards, if no longer supplied from the surrounding medium. The finding by Heber et al. (1997) that the He I line at 4471 Å has been weaker by a factor of two or three for several months would fit into this scenario, if we assume that HS 0209+0832 travels through an inhomogeneous medium.

Key words: stars: abundances – stars: atmospheres – stars: individual: HS 0209+0832 – stars: white dwarfs – ultraviolet: stars

* Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal ID 7473.

Send offprint requests to: B. Wolff

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000

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