The Evolutionary Connection Between S and C Stars: Evidence from Star Clusters and the Magellanic Clouds

Abstract

Observations of intermediate-age clusters in the Magellanic Clouds indicate an evolutionary sequence, M→S→C, for stars on the asymptotic giant branch. Trends with age of the clusters are generally in agreement with theoretical prediction but the lifetime of the S star stage appears to be longer than expected. CS stars have been found among high luminosity field variables and can be accounted for by the envelope-burning process. The presence of faint S stars in ω Cen, where they may be interpreted as the result of a high primordial heavy-element abundance, shows that S stars are not necessarily all self-enriched in s-process material, while the low luminosity J stars have probably evolved up the giant branch as carbon stars without passing through an S star stage.