Astron. Astrophys. 334, 618-632 (1998)

Spectral analyses of PG 1159 stars: constraints on the GW Virginis pulsations from HST observations ^*

S. Dreizler ^{1, 2, 3} and U. Heber <sup>3

1</sup> Institut für Astronomie und Astrophysik, Universität Tübingen, Waldhäuser Strasse 64, D-72076 Tübingen, Germany
² Institut für Astronomie und Astrophysik der Universität, D-24098 Kiel, Germany
³ Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstrasse 7, D-96049 Bamberg, Germany

Received 23 December 1997 / Accepted 24 February 1998

Abstract

We present the results of a quantitative analysis of UV and optical spectra of nine PG 1159 stars, very hot hydrogen-deficient [pre-] white dwarfs, by means of line blanketed NLTE model atmospheres. Four programme stars constitute the GW Vir variables, a class of non-radial g-mode pulsators. Precise effective temperatures, carbon, nitrogen and oxygen abundances and spectroscopic masses are used to constrain the GW Vir pulsations. The blue edge of the instability strip is at 140 000 K (PG 1159-035). PG 0122 200 sets the red edge at 80 000 K, but is also one of the coolest PG 1159 stars known, suggesting that the pulsations are stopped when the transformation of a PG 1159 star into a hot white dwarf occurs by gravitational settling of the metals. Four non-variables are found to lie inside the GW Vir instability strip indicating that an additional parameter determines whether a PG 1159 star pulsates. Abundances of C and O in the pulsating stars appear to be higher than in the non-variables in agreement with the theoretical prediction that the pulsations are driven by cyclic ionization of C and O. The outstanding discovery of our investigation, however, is a strong correlation between the nitrogen abundance and pulsations. All GW Vir stars are nitrogen rich, whereas no nitrogen can be detected in the non-variables except in PG 1144 005. We conjecture that this correlation provides a key for the understanding of the driving mechanism.

Comparing their position in the - log g diagram to new evolutionary models we conclude that most programme stars are post-AGB stars of rather low mass (0.5 ... 0.65 ) which have lost their entire hydrogen-rich envelope and part of their helium-rich envelope whereas we confirm HS 0704 6153 to be an AGB manqué star. The high nitrogen abundance in four stars is a tracer of mixing processes which have led to ingestion and burning of hydrogen during the final helium shell flash.

Key words: stars: abundances stars: atmospheres stars: evolution stars: oscillations stars: AGB and post-AGB stars: white dwarfs

* Based on observations obtained a) with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555; b) at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy; c) at the European Southern Observatory, La Silla, Chile; d) with the International Ultraviolet Explorer (IUE).

Send offprint requests to: S. Dreizler

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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