ON THE DRIVING MECHANISM AND THE COEXISTENCE OF VARIABLE AND NONVARIABLE STARS IN THE DOMAIN OF THE PULSATING PG 1159 STARS

摘要

The fact that we find pulsating and nonpulsating stars mixed together in the PG 1159 region of the log g - Teff diagram has been a long standing puzzle. The poor understanding of the driving mechanism in those stars has been the reason why it has taken so long to address properly this problem. Following the work of Saio (1996) and Gautschy (1997) based on the OPAL opacities, Quirion, Fontaine, & Brassard (2004) recently showed that we are now able to understand and reproduce the ranges of observed periods in the pulsating PG 1159 stars in terms of the original K-mechanism associated with the partial ion-ization of the K-shell electrons of C and O which, along with He, make up the composition of the envelope of those stars. Contrary to others, those three studies agree in that no composition gradients are needed between the atmospheric layers and the driving region. Furthermore, the cohabitation of pulsating and nonpulsating PG 1159 stars is naturally explained in terms of a dispersion in atmospheric parameters and in terms of a variation in surface composition from star to star. In particular, the most He-rich stars tend to be stable. We briefly return to some of the findings discussed by Quirion et al. (2004) in this paper, but we also present the results of additional calculations aimed at describing better the role of the chemical composition (in particular the role of metallicity) as well as that of the stellar mass on the boundaries of the instability domain in the log g - Teff plane.