1998A&A...334..618D

We present the results of a quantitative analysis of UV and optical spectra of nine PG1159 stars, very hot hydrogen-deficient [pre-] white dwarfs, by means of line blanketed NLTE model atmospheres. Four programme stars constitute the GWVir 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 GWVir pulsations. The blue edge of the instability strip is at 140000K (PG1159-035). PG0122+200 sets the red edge at 80000K, but is also one of the coolest PG1159 stars known, suggesting that the pulsations are stopped when the transformation of a PG1159 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 PG1159 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 GWVir stars are nitrogen rich, whereas no nitrogen can be detected in the non-variables except in PG1144+005. We conjecture that this correlation provides a key for the understanding of the driving mechanism. Comparing their position in the T_eff-logg diagram to new evolutionary models we conclude that most programme stars are post-AGB stars of rather low mass (0.5 ... 0.65 M_☉) which have lost their entire hydrogen-rich envelope and part of their helium-rich envelope whereas we confirm HS0704+6153 to be an AGB manque 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.