The importance of EUV spectra of white dwarfs

It is clear, from chapters 3 and 4 (sections 3.6, 3.7, 4.3.2), that the ROSAT and EUVE sky surveys have made significant contributions to our understanding of the physical structure and evolution of white dwarfs. Among the most important discoveries are the ubiquitous presence of heavy elements in the very hottest DA stars (above 40 000–50 000 K), the existence of many unsuspected binary systems containing a white dwarf component and a population of white dwarfs with masses too high to be the product of single star evolution. In each case, however, the detailed information that could be extracted from the broadband photometric data was often rather limited. For example, although simple photospheric models (e.g. H+He) could often be ruled out, it was not possible to distinguish between more complex compositions with varying fractions of He and heavier elements. Furthermore, rather simplistic assumptions needed to be made about the relative fractions of the H and He in the interstellar medium besides the degree of ionisation of each element, to restrict the number of free parameters to a tractable level in any analysis. Direct spectroscopic observations of gas in the LISM (see sections 7.3 and 7.6) indicate that the convenient assumption of a cosmic He/H ratio (0.1) and minimal ionisation is unlikely to be reasonable.

Spectroscopic observations of white dwarfs in the EUV can address a number of important questions.