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

Abstract

In four globular clusters (GCs) a non negligible 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 $\omega$ Cen and NGC 2808, and from the 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$\simgt$0.35, and it is difficult to understand how it can be produced without any --or, for $\omega$ 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 $\simeq$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 into the clusters, thanks to their small supernova natal kicks.