Identification of Ne VIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars

K. Werner; T. Rauch; J. W. Kruk

doi:doi:10.1051/0004-6361:20078152

Free access

Issue		A&A Volume 474, Number 2, November I 2007


Page(s)		591 - 597
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20078152

A&A 474, 591-597 (2007)
DOI: 10.1051/0004-6361:20078152

Identification of Ne VIII lines in H-deficient (pre-) white dwarfs: a new tool to constrain the temperature of the hottest stars

K. Werner¹, T. Rauch¹, and J. W. Kruk²

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: werner@astro.uni-tuebingen.de
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

(Received 25 June 2007 / Accepted 16 August 2007)

Abstract
For the first time, we have identified $\ion{Ne}{viii}$ absorption lines in far-UV spectra of the hottest known ( $T_{\rm eff}$ $\ga$ 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 $\ion{Ne}{viii}$ features and not, as hitherto suspected, as ultrahigh ionised $\ion{O}{viii}$ 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 ( $\ion{O}{vii-viii}$ , $\ion{C}{v-vi}$ ) must be revised. These lines can be entirely attributed to thermally excited species ( $\ion{Ne}{vii-viii}$ , $\ion{N}{v}$ , $\ion{O}{vi}$ ). Photospheric $\ion{Ne}{viii}$ lines are also identified in the hottest known He-rich white dwarf (KPD 0005+5106), some of which were also attributed to $\ion{O}{viii}$ previously. This is a surprise because it must be concluded that KPD 0005+5106 is much hotter ( $T_{\rm eff}$ $\approx$ 200 000 K) than hitherto assumed ( $T_{\rm eff}$ $\approx$ 120 000 K). This is confirmed by a re-assessment of the $\ion{He}{ii}$ 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

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