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Abteilung Astronomie

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Preprint E/04

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

You-Hua Chu (1,2), Robert A. Gruendl (1), Rosa M. Williams (1,2), Theodore R. Gull (2,3), Klaus Werner (4)

(1) Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West 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.
(3) Goddard Space Flight Center, NASA, Code 681, Greenbelt, MD 20771-5302
(4) Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany

To be published in: Astronomical Journal

Abstract. We have detected an ionized nebula around the hot DO white dwarf KPD 0005+5106, and used the [O-III]/H-alpha 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 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 Msun, 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 LX/Lbol 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 as neither photospheric emission at such high energies nor a high-temperature corona is expected from current stellar atmospheric models 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.
Key words: white dwarfs - stars: individual (KPD 0005+5106) - planetary nebulae: general - H-II regions

Preprint (1,62 Mb PDF file including figures)

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