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

Abstract

We report on the detection of two faint blue stars (HE0504-2408 and HS0713+3958) discovered by the Hamburg Schmidt surveys whose spectra are dominated by broad and shallow metal absorption lines from ultrahighly excited ionization stages (CV, CVI/NVI, NVII/OVII, OVIII, NeIX, NeX) 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 70000K) causing the observed HeII line spectrum with symmetric profiles. However, the HeII 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.