FUSE Spectroscopy of the Two Prototype White Dwarfs with Signatures of a Super-Hot Wind

The O VIII phenomenon describes the occurrence of ultra-high ionization absorption lines of the CNO elements (e.g. O VIII, N VII, C VI, and even Ne X) in the optical spectra hot of DO WDs. This requires temperatures of almost 106 K. Since the first discovery of two such objects (Werner et al. 1995) we realized that 50% of all DO WDs are affected (Dreizler et al. 1995), hence it could be that all DO WDs experience this phenomenon at the beginning of the WD cooling sequence. The HeII profiles are symmetric, hence line formation occurs in hydrostatic layers. However, we cannot uniquely fit the unusually strong profiles with any model atmosphere, preventing precise parameter determinations. The optical high ionization CNO lines have pronounced blue wings, corresponding to outflow velocities of up to several thousand km/s. We stress that none of the objects was detected in the X-ray region. The inferred low X-ray luminosity confirms that we do not see absorption lines from super-hot photospheres, but that the high temperatures are possibly localized along shock fronts in the wind.