Jeffrey W. Kruk(1) and Klaus Werner(2)
(1)Department of Physics and Astronomy, Johns Hopkins University,
Baltimore, MD 21218, U.S.A.
(2)Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
1998, Astrophysical Journal, 502, 858
Abstract. We present for the first time systematic FUV spectroscopy of five representative PG 1159 stars (H 1504+65, K 1-16, PG 1424+535, PG 1707+427, PG 1159-035) which extends down to the hydrogen Lyman edge. In addition we have observed the hottest known DO white dwarf (KPD 0005+5106) and the hot hydrogen-rich central star of the PN Sh 2-216. The medium resolution spectra were taken with the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission in 1995. We have performed line identifications and NLTE model atmosphere fits. The results confirm earlier analyses of optical and UV data and complement recent HST/GHRS UV spectroscopy.
The following conclusions are drawn from the HUT data. The absence of absorption lines from the He II Balmer series finally proves the helium-deficiency in H 1504+65 and confirms its nature as a naked C/O stellar core. The detection of a P Cygni profile from the O VI resonance line in the spectrum of the central star of K 1-16 allows a first reliable determination of the mass-loss rate from a PG 1159 star. It suggests that the wind is radiation driven. The spectra of the pulsator/non-pulsator pair PG 1707+427 and PG 1424+535 reveal differences in the photospheric parameters, which could not be detected with optical spectroscopy. In particular, we announce the detection of nitrogen in the pulsator PG 1707+427, which is a remarkable phenomenon among PG 1159 stars. Also, in the prototype itself we find hints for photospheric nitrogen. The hot DO KPD 0005+5106 exhibits H_2 absorption lines similar to those found in the spectrum of the central star Sh 2-216. This may signify the presence of an old planetary nebula surrounding the DO star.
Key words: line: identification - stars: abundances - stars: AGB and post-AGB - ultraviolet: stars - white dwarfs
Paper (405k gzip'ed Postscript including figures)
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