A theoretical analysis of pulsation driving in PG 1159 stars
- XTA, MS B220, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
- Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw (Poland)
Our understanding of stars of the PG 1159 spectral type is not yet satisfactory, in spite of the recent success of asteroseismology. Kawaler and coworkers match the observed pulsation frequencies of PG 1159{minus}035 and PG 2131+066 quite well with evolutionary models, but they fail to identify the mechanism exciting their pulsations. Stanghellini, Cox, & Starrfield show that the classical {kappa}, {gamma} mechanism acting in the C/O partial ionization zone can excite certain {ital g}-modes but requires compositions that seem unrealistic. Here we study the impact of the new OPAL opacities on the conditions required to drive the modes observed in the PG 1159 spectral class stars. To this end, we present the nonadiabatic pulsation results of a parametric survey of quasi-evolutionary models of PG 1159 pre-white dwarfs. We examine the effect of varying the chemical composition of the driving region, the stellar radius, and stellar mass on the location of the instability strip and the maximum unstable period. Changes in the oxygen mass fraction of the driving region and the stellar radius have a strong effect on the predicted spectrum of unstable modes. We do not find unstable modes with periods longer than 150 s unless the driving region, located near 10{sup {minus}9} {ital M}{sub {asterisk}}, has at least 50{percent} oxygen. The maximum unstable period increases by factors of 2{endash}3 when we increase the radius of our models by 40{percent}{endash}50{percent}. Decreasing the stellar mass also increases the radius, and the maximum unstable period increases from {approximately}300{endash}400 s at 0.65 {ital M}{sub {circle_dot}} to {approximately}800 s at 0.50 {ital M}{sub {circle_dot}} for models with 50:50 C/O cores. Based on these results, we suggest that no pulsating PG 1159 star has a driving region with photospheric abundances; rather they are probably oxygen-rich. (Abstract Truncated)
- OSTI ID:
- 285532
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 462; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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