The role of super-asymptotic giant branch ejecta in the abundance patterns of multiple populations in globular clusters

Abstract

In order to account for the chemical composition of a stellar second generation (SG), Globular Clusters (GCs) evolution models based on the asymptotic giant branch (AGB) scenario so far included only the yields available for the massive AGB stars, while the possible role of super-AGB ejecta was either extrapolated or not considered. In this work, we explore the role of super-AGB ejecta using yields recently calculated by Ventura and D'Antona. Models of clusters showing extended Na-O anticorrelations, like NGC 2808, indicate that a SG formation history similar to that outlined in our previous work is required: formation of an Extreme population with very large helium content from the pure ejecta of super-AGB stars, followed by formation of an Intermediate population by dilution of stellar ejecta with pristine gas. The very O-poor Na-rich Extreme stars can be accounted for once deep-mixing is assumed in SG giants forming in a gas with helium abundance Y> 0.34, which significantly reduces the atmospheric oxygen content, while preserving the sodium abundance. On the other hand, for clusters showing a mild O-Na anticorrelation, like M 4, the use of the new yields broadens the range of SG formation routes leading to abundance patterns consistent with observations. It is shown that models in which SG stars form only from super-AGB ejecta promptly diluted with pristine gas can reproduce the observations. We discuss the variety of small helium variations occurring in this model and its relevance for the horizontal branch morphology. In some of these models the duration of the SG formation episode can be as short as \sim10 Myr; the formation time of the SG is thus compatible with the survival of a cooling flow in the GC, previous to the explosion of the SG core collapse supernovae. We also explore models with formation of multiple populations in individual bursts, each lasting no longer than \sim10 Myr.