Extreme Ultraviolet Spectroscopy of White Dwarfs

M.A. Barstow1, J.B. Holberg2, D. Koester3, J.A. Nousek4, K. Werner3

1Department of Physics and Astronomy, University of Leicester, University ROAD, Leicester LE1 7RH, UK
2Lunar and Planetary Laboratory of Arizona, Tucson AZ, USA
3Institut für Astronomie und Astrophysik der Universität, D-24098 Kiel, Germany
4Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA, USA

It has long been predicted that Extreme Ultraviolet (EUV) observations of white dwarfs would be one of the principal keys to understanding the composition and structure of white dwarf atmospheres. Indeed, the ROSAT X-ray and EUV sky survey has revealed that most H-rich white dwarfs hotter than 40,000K contain significant quantities of heavy elements in their atmospheres. However, the ROSAT data are unable to say much about the role of helium in the hottest white dwarfs, which remains an important question. Spectroscopic observations with the Extreme Ultraviolet Explorer allow a direct and sensitive search for HeII absorption to be performed. Furthermore, they potentially reveal the detailed nature of the sources of EUV opacity. In most cases examined no trace of He is found but HZ43 and G191-B2B show tantalising evidence for the presence of He, which would have important implications for our views of white dwarf evolution.

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