Evolution of AGB stars

Summary

Abstract

Observational and theoretical estimates for mass loss rates from AGB stars are discussed. Then models for the evolution of AGB stars including mass loss and the effects of helium shell flashes are presented. Finally, the possibility of mass loss by binary mass transfer is discussed.

Introduction

It is well established that the bulk of mass loss from low and intermediate mass stars occurs during the asymptotic giant, branch (AGB) stage of evolution, leading to the well-defined sequence of mass-losing stars in the IRAS two-colour diagram (van der Veen and Habing 1988) and the formation of planetary nebulae (Abell and Goldreich 1966; Renzini 1981). However, a reliable theoretical understanding of the causes of mass loss is still not available, although progress is being made. An additional complication is that the time history of mass loss during AGB evolution is quite complex since AGB evolution is modulated by helium shell flashes which control the surface luminosity and thereby the mass loss rate. In this paper, mass loss rates from AGB stars are discussed and the effects of helium shell flashes on the mass loss are described and compared with observations. Time dependent winds produced by AGB stars are reviewed. Finally, the evolution of AGB stars that lose mass in binary mass transfer events is briefly described.

AGB mass loss rates

IRAS observations of stars in the solar neighbourhood indicate that those stars with substantial circumstellar shells - those with high mass loss rates - are nearly all AGB stars undergoing large-amplitude pulsation (Habing 1990).