The Nebular Environment and Enigmatic Hard X-Ray Emission of the Hot DO White Dwarf KPD 0005+5106

We have detected an ionized nebula around the hot DO white dwarf KPD 0005+5106 and used the [O III]/Hα ratios and nebular velocities to separate this nebula from the background H II region of AO Cas. The angular size of the [O III] nebula of KPD 0005+5106 is ∼3°. The velocity of this nebula is similar to those of the local interstellar H I gas and the interstellar/circumstellar absorption lines in the UV spectra of KPD 0005+5106 but has a large offset from those of the stellar photospheric lines. The mass of the ionized nebula, ∼70 M ⊙ , indicates that it consists of interstellar material and that the velocity offset between the star and the ambient medium should not be interpreted as a wind outflow. We have also analyzed the ROSAT PSPC observation of KPD 0005+5106 to determine its hard X-ray luminosity. Using the L X / L bol relation for late-type stars and the lack of obvious near-IR excess of KPD 0005+5106, we exclude the possible existence of a binary companion with coronal activity. Without a wind outflow, the presence of stellar O VIII emission requires that X-rays at energies greater than 0.871 keV are present in the vicinity of KPD 0005+5106. This hard X-ray emission is most puzzling, since neither photospheric emission at such high energies nor a high-temperature corona is expected from current stellar atmospheric models of KPD 0005+5106. X-ray observations with high angular resolution and sensitivity are needed to confirm the positional coincidence and to enable X-ray spectral analyses for constraining the physical origin of the hard X-ray emission from KPD 0005+5106.