Self-Enrichment in Globular Clusters: Is There a Role for the Super-Asymptotic Giant Branch Stars?

Abstract

In four globular clusters (GCs) a nonnegligible fraction of stars can beinterpreted only as a very helium-rich population. The evidence comesfrom the presence of a ``blue'' main sequence in {$ω$} Cen and NGC2808, and from the very peculiar horizontal-branch morphology in NGC6441 and NGC 6388. Although a general consensus is emerging on the factthat self-enrichment is a common feature among GCs, the helium contentrequired for these stars is Y{\gt}~0.35, and it is difficult tounderstand how it can be produced without any-or, for {$ω$} Cen,without a considerable-associated metal enhancement. We examine thepossible role of super-AGB stars, and show that they may provide therequired high helium. However, the ejecta of the most massive super-AGBsshow a global CNO enrichment by a factor of ~=4, due to the dredge-outprocess occurring at the second dredge-up stage. If these clusters showno evidence for this CNO enrichment, we can rule out that at least themost massive super-AGBs evolve into O-Ne white dwarfs and take part inthe formation of the second-generation stars. This latter hypothesis mayhelp to explain the high number of neutron stars present in GCs. Themost massive super-AGBs would in fact evolve into electron-capturesupernovae. Their envelopes would be easily ejected out of the cluster,but the remnant neutron stars remain in the clusters, thanks to theirsmall supernova natal kicks.