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

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5. Summary

We derived precise effective temperatures and chemical compositions of nine PG 1159 stars from UV and optical spectra. Four programme stars belong to the GW Vir variables, a class of non-radial g-mode pulsators. Our results define observationally determined boundaries of the GW Vir instability strip in the [FORMULA] - log g diagram which can be used as constraints for theoretical pulsation models. The blue edge is at 140 000 K defined by PG 1159-035 and the red edge lies at 80 000 K (PG 0122 [FORMULA] 200). The cool edge of the instability strip coincides with the coolest PG 1159 stars indicating that the pulsations are stopped by the transition process towards DO white dwarfs. However, four non-pulsating PG 1159 stars are located within this strip necessitating an additional parameter which determines whether a PG 1159 stars does pulsate. Abundances of C and O in the pulsating stars appear to be slightly 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 abundances and pulsations. All GW Vir stars are nitrogen rich, whereas no nitrogen can be detected in the non-variables. We conjecture that this correlation provides a key for the understanding of the driving mechanism. PG 1144 [FORMULA] 005, the only other known PG 1159 star with a high N abundance, is somewhat hotter and of lower gravity than our programme stars, and was not found to vary. In view of the above correlation it is worthwhile to reobserve PG 1144 [FORMULA] 005 photometrically.

The nitrogen abundance is also a tracer for the evolutionary history of these stars. In order to have nitrogen in an extremely carbon and oxygen rich surrounding mixing processes leading to hydrogen ingestion and burning during a late helium shell flash are required.

The mass range for compact GW Vir variables seems to be very narrow ([FORMULA] 0.55 [FORMULA] [FORMULA]). The sample contains one Planetary Nebula nucleus, PG 1520 [FORMULA] 525, which appears to be somewhat more massive (0.65 [FORMULA]) than the rest of the sample.

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998