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R. Napiwotzki, S. Jordan, S. Bowyer, M. Hurwitz, D. Koester, T. Rauch, V. Weidemann
We present the results of a recent spectroscopic investigation of the cool DO white dwarf HD 149499 B in the EUV and FUV ranges. Observations were performed with the spectrograph of the EUVE satellite and the Berkeley EUV/FUV spectrometer of the ORFEUS space experiment. (Based on the development and utilization of ORFEUS -- Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometers, a collaboration of the Astronomical Institute of the University of Tuebingen, the Space Astrophysics Group, University of California, Berkeley, and the Landessternwarte Heidelberg.) The analysis of the ORFEUS spectrum, performed with a grid of LTE model atmospheres, yielded the basic parameters T_eff = 49500 +/- 500 K and log g = 7.97 +/- 0.08. This result is confirmed by the EUVE spectra. The photospheric hydrogen Lyman lines in the FUV spectrum indicate the presence of hydrogen: log n_H/n_He = -0.65 +/- 0.12. The implications of this finding for the spectral evolution of white dwarfs are discussed. A check of the LTE assumption was performed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar hydrogen column towards the HD 149499 system amounts to N(H) = (7 +/- 2)E18 cm^(-2).
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