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

In four globular clusters (GCs) a nonnegligible fraction of stars can be interpreted only as a very helium-rich population. The evidence comes from the presence of a "blue" main sequence in ω Cen and NGC 2808, and from the very peculiar horizontal-branch morphology in NGC 6441 and NGC 6388. Although a general consensus is emerging on the fact that self-enrichment is a common feature among GCs, the helium content required for these stars is Y≳ 0.35, and it is difficult to understand how it can be produced without any—or, for ω Cen, without a considerable—associated metal enhancement. We examine the possible role of super-AGB stars, and show that they may provide the required high helium. However, the ejecta of the most massive super-AGBs show a global CNO enrichment by a factor of ≃4, due to the dredge-out process occurring at the second dredge-up stage. If these clusters show no evidence for this CNO enrichment, we can rule out that at least the most massive super-AGBs evolve into O-Ne white dwarfs and take part in the formation of the second-generation stars. This latter hypothesis may help to explain the high number of neutron stars present in GCs. The most massive super-AGBs would in fact evolve into electron-capture supernovae. Their envelopes would be easily ejected out of the cluster, but the remnant neutron stars remain in the clusters, thanks to their small supernova natal kicks.