Ultra-high excitation features in hot white dwarfs: A DAO and three DO stars discovered by the Hamburg-Schmidt survey.

Abstract

We report on the discovery of four hot white dwarfs, one DAO (HS2115+1148) and three DO (HS0158+2335, HS 0727+6003 and HS 2027+0651). The spectra of these stars exhibit 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. We propose that they are formed in a stellar wind, explaining their asymmetric shape. However, the He II line spectra of the three DOs as well as the Balmer-line spectrum of the DAO are peculiar and cannot be fitted by static NLTE model atmospheres. These stars increase the number of hot degenerate stars showing this phenomenon to six, this is the first time it is observed in a hydrogen-rich star. Thus, these six stars might constitute a new spectral subclass of hot white dwarfs suffering mass-loss from a hot and fast wind.