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 KPD0005+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 KPD0005+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 UV spectra of KPD0005+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 KPD0005+5106 to determine its hard X-ray luminosity. Using the LX/Lbol relation for late-type stars and the lack of obvious near-IR excess of KPD0005+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 KPD0005+5106. This hard X-ray Department of Astronomy, University of Illinois at Urbana-Champaign, 1002West Green Street, Urbana, IL 61801 2 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the National Science Foundation. Goddard Space Flight Center, NASA, Code 681, Greenbelt, MD 20771-5302 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany