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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint B/98

FUV Spectroscopy of PG 1159 Stars with the Hopkins Ultraviolet Telescope

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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