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

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 absorption lines in far-UV spectra of the hottest known (T_eff  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 (T_eff ≈ 200 000 K) than hitherto assumed (T_eff ≈ 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.