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 e f f 60 000 K) DA white dwarf PG1342+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-A O V I lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O V I 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 e f f 67 000 and 54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C I I I multiplet near 1176 A, 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.

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