EUV observations of opacities along the lines of sight to and in the photospheres of hot hydrogen rich white dwarfs

EUV spectral observations provide an excellent means with which to examine the composition and physical state of gas along the line of sight to and in the atmospheres of hot hydrogen rich white dwarfs. Here I present the results of an analysis of a large number of spectra of hot DA type white dwarfs in the temperature range 25000K ≲ Teff ≲ 55000K obtained with the Extreme Ultraviolet Explorer satellite. No evidence for the presence of photospheric helium is found in the spectra of any of the sample objects, adding to the growing body of observations which argues against the one channel evolutionary hypothesis of Fontaine & Wesemael (1987). Furthermore, it is found that contrary to theoretical predictions and the results of earlier photometric studies, the majority of the opacity of EUV radiation observed towards those DAs with 40000K ≲ Teff ≲ 50000K is provided by HI, HeI and HeII along the line of sight as opposed to photospheric heavy metals. The 228A Lyman edge of HeII is detected in the spectra of 6 of the sample objects allowing a direct measurement of the line of sight averaged ionization fractions of hydrogen and helium. The ionization fractions towards these 6 stars can be consistent with a conclusion of a uniformly ionized ISM with weighted means of fH = 0.37 +/- 0.1 fHe = 0.28 +/- 0.04. The limits placed on the fractions towards the remaining sample objects do not contradict such a conclusion. The observed high level of helium ionization can be consistent with a model in which the blast from a nearby supernova shock ionized the gas of the LISM some million or so years ago. The most peculiar line of sight characteristics towards the DAO+dM binary RE J0720-318 revealed by the unusually large HeI edge at 504A(e.g. fH ~ 0.9; Burleigh, Barstow & Dobbie 1997), has been attributed to the presence of an ionized cloud lying along this line of sight at a distance of 123-170 pc.