Discovery of two hot DO white dwarfs exhibiting ultrahigh- excitation absorbtion lines

K. Werner1, S.Dreizler2, U.Heber2, T.Rauch1, L. Wisotzki3, H.-J. Hagen3

1Institut für Astronomie und Astrophysik der Universität, D-24098 Kiel, Germany
2Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstr. 7, D-96049 Bamberg, Germany
3Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany

We report on the detection of two faint blue stars (HE 0504-2408 and HS 0713+3958) discovered by the Hamburg Schmidt surveys whose spectra are dominated by broad and shallow metal absorption lines from ultrahighly excited ionization stages (C V, C VI, N VI, N VII, O VII, O VIII, Ne IX, Ne X) with ionization energies ranging from 392eV to 1362eV. Detailed non-LTE modeling excludes a photospheric origin of these lines. Instead we propose that they are formed in a stellar wind, explaining their asymmetric shape. The underlying stellar source is probably a hot helium-rich white dwarf ( Teff about 70000 K) causing the observed He II line spectrum with symmetric profiles. However, the He II absorptions are relatively strong and cannot simply be reproduced by a model atmosphere for a DO white dwarf.

Thus, these two stars might constitute a new spectral subclass of hot helium-rich white dwarfs exhibiting spectral signatures never observed before in any astrophysical object.

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