Temperature determination of the cool DO white dwarf HD 149499B from EUVE observations

We present a LTE model atmosphere analysis of the EUVE medium and long wavelength spectrum of the cool DO white dwarf HD149499B. This observation in the spectral range 230-700°A supplements a previous analysis of an ORFEUS spectrum between 912 and 1170°A which yielded the atmospheric parameters Teff = 49500 ± 500K and log g = 7.97 ± 0.08 and a hydrogento- helium number ratio of 0.22 ± 0.11. The EUVE data are in full agreement with the ORFEUS result and allow a more precise determination of the effective temperature (Teff = 49500 ± 200K) and the interstellar hydrogen column density (NH = 7 · 1018 cm−2). None of the features in the EUVE spectrum could be identified with any additional absorber besides helium. Exploratory calculations with fully metal blanketed LTE model atmospheres show that the metal abundances predicted by current diffusion theory are clearly at variance with the observed spectrum.