We have computed evolutionary sequences for the asymptotic-giant-branch phase of low-mass solar-metallicity stars in order to study a radiation-pressure instability for ejecting a planetary nebula (PN). Our results show that the ratio beta of gas to total pressure at the base of the hydrogen envelope drops abruptly during a helium-shell flash as the envelope expands and the surface luminosity increases. We find that the minimum value of beta decreases from one flash to the next and eventually approaches 0.0, implying that the entire envelope is then supported by radiation pressure. At this point the luminosity at the base of the envelope reaches the Eddington limit, and the star will most likely eject its hydrogen envelope. The critical envelope mass at PN ejection is found to be a linear function of the core mass M_H, varying from ~ 0.04 Msun at M_H = 0.64 Msun to ~ 1.04 Msun at M_H = 0.91 Msun for a solar metallicity. This critical envelope mass decreases considerably at lower metallicities, implying a possible metallicity dependence of the PN luminosity function. The minimum value of beta during a flash occurs at the time when carbon dredge-up is most likely. We suggest that such dredge-up might trigger PN ejection by increasing the opacity and thereby lowering the Eddington luminosity at the base of the envelope. This might explain why the majority of halo PN's, including K648 in M15, as well as the non-Type I PN's in the Magellanic Clouds are carbon rich.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1997