FUSE observations of PG1342+444:: new insights into the nature of the hottest DA white dwarfs

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (T(eff) approximate to 60 000 K) DA white dwarf PG 1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Angstrom Ovi lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG 1159 stars and so-called Ovi central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (T(eff) approximate to 67 000 and approximate to 54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the CIII multiplet near 1176 Angstrom, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.