Identification of Ne VIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars *,**
Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: email@example.com
2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
Accepted: 16 August 2007
For the first time, we have identified absorption lines in far-UV spectra of the hottest known (Teff 150 000 K) hydrogen-deficient (pre-) white dwarfs of spectral type PG1159. They are of photospheric origin and can be matched by synthetic non-LTE line profiles. We also show that a number of UV and optical emission lines in these stars can be explained as being photospheric features and not, as hitherto suspected, as ultrahigh ionised lines created along shock-zones in the stellar wind. Consequently, we argue that the long-standing identification of the same emission lines in hot [WR]-type central stars as being due to ultrahigh-ionised species (, ) must be revised. These lines can be entirely attributed to thermally excited species (, , ). Photospheric lines are also identified in the hottest known He-rich white dwarf (KPD 0005+5106), some of which were also attributed to previously. This is a surprise because it must be concluded that KPD 0005+5106 is much hotter (Teff ≈ 200 000 K) than hitherto assumed (Teff ≈ 120 000 K). This is confirmed by a re-assessment of the line spectrum. We speculate that the temperature is high enough to explain the mysterious, hard X-ray emission (1 keV) as being of photospheric origin.
Key words: stars: abundances / stars: atmospheres / stars: evolution / stars: AGB and post-AGB / white dwarfs
Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.
Some of the data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts.
© ESO, 2007