2000A&A...361..629W

The DAB white dwarf HS 0209+0832 is one of the few white dwarfs with detected helium in the DB gap between 28000 and 45000K. We have obtained ultraviolet and optical spectra with the Hubble Space Telescope (HST). The analysis results in an effective temperature of about 35000K and a helium abundance of about 1%. The presence of CIV at 1548Å, 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 (≃10^–16M_☉) 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 HeII 1640Å 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. (1997A&A...324L..53H) that the HeI line at 4471Å 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.