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M.A. Barstow (University of Leicester), K. Werner (Universitat Tubingen)
White Dwarfs are among the oldest objects in the Galaxy. As the remnant of all stars with initial masses less than 8M(solar), they provide important laboratories for the study of evolutionary processes and the behaviour of matter at extremes of temperature and density. Study of their space density and luminosity distribution helps map out the history of star formation and can, in principle, determine the age of the galactic disk. Recently it has been suggested that cool white dwarfs may account for a substantial fraction of the missing mass in the galactic halo. However, any of these results depend on our understanding of white dwarf evolution and, in particular on predictions of the cooling rates. These are in turn affected by the mass, radius and photospheric composition of the stars.
Extreme ultraviolet observations of white dwarf stars have made a profound contribution to our understanding of white dwarf composition. The emergent EUV flux is highly sensitive to the presence of small traces of heavy elements. Hence, the photometric surveys were able to establish the general pattern of heavy element abundances across the white dwarf sample, while follow-up spectroscopy has provided more detailed measurements of photospheric composition of individual stars. A clear indication of the tremendous importance of access to EUV data is the contrast in the formation depths of lines in this region, compared to the UV band, which allows us to determine the depth dependence of the white dwarf composition.
However, the limited spectral resolution available until recently has made it impossible to distinguish absorbing material lying within the stellar photosphere from possible circumstellar or interstellar components. A new generation of EUV instrumentation will allow these problems to be solved. We consider the scientific importance of such developments and assess their impact on the study of white dwarfs.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.