1997A&A...320..811G

To improve our understanding of the pulsational instabilities found in some of the PG 1159-type pre-white dwarfs we performed detailed nonradial stability analyses on helium-star evolutionary models. Adopting canonical chemical compositions as derived from spectroscopic observations, we found pulsational instabilities for all stellar masses considered (0.57, 0.63, 0.7M_☉). The blue boundary of the instability domain agrees satisfactorily with observations. The red edge, on the other hand, is found to extend to lower temperatures than presently admitted by observations. The dependence of the pulsational instabilities on helium or even hydrogen abundance in the stellar envelopes is found to be less stringent than emphasized hitherto. In particular, we obtained pulsationally unstable g modes for model stellar envelopes which are representative for HS 2324+3944 - a newly discovered PG 1159-star with a hydrogen abundance X=0.2 and which is claimed to be pulsating.