Massive AGB models of low metallicity: the implications for the self-enrichment scenario in metal-poor globular clusters

Abstract

Context: We present the physical and chemical properties of intermediate-mass stars models of low metallicity, evolved along the thermal pulse phase. Aims: The target of this work is to extend to low metallicities, Z=1,2 and 6 x 10^{-4}, the models previously computed for chemistries typical of Globular Clusters of an intermediate metallicity (Z=0.001), and for the most metal-rich clusters found in our Galaxy (Z=0.004); the main goal is to test the self-enrichment scenario also for metal poor Globular Clusters Methods: We calculated three grids of intermediate-mass models with metallicities Z=10^{-4}, 2x10^{-4}, and 6x10^{-4}; the evolutionary sequences are followed from the pre-main sequence throughout the AGB phase, almost until the ejection of the whole envelope. We discuss the chemistry of the ejecta, and in particular the mass fractions of those elements that have been investigated during the many, deep, spectrocopic surveys of Globular Clusters Results: Although the data for oxygen and sodium are scarce for low metallicity Globular Clusters, the few data for the unevolved stars in NGC6397 are compatible with the models. Further, we find good agreement with the C--N anticorrelation of unevolved stars in the cluster M15. In this cluster, however, no stars having low oxygen ([O/Fe] = -1) have been detected. The most massive, very metal poor clusters, should contain such stars, according to the present models. At the lowest metallicity Z=10^{-4}, the ejecta of the most massive AGBs have C/O>1, due to the dramatic decrease of the oxygen abundance. We discuss the possible implications of this prediction.