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

INAF - Observatory of Rome, via Frascati 33, 00040 MontePorzio Catone (RM), Italy e-mail: [ventura;dantona]@oa-roma.inaf.it

Received: 13 October 2008
Accepted: 16 February 2009

Abstract

Context. We present the physical and chemical properties of intermediate-mass star models of low metallicity, as they evolve along the thermal pulse phase.

Aims. We extend to low metallicities, of Z =1, 2 and 610^-4, models previously computed for chemical compositions 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). We aim to test in particular the self-enrichment scenario for metal-poor globular clusters.

Methods. We calculated three grids of intermediate-mass models with metallicities Z = 10^-4, 210^-4, and 610^-4, following their evolutionary sequences from the pre-main-sequence to the asymptotic giant branch phase, almost until the ejection of the entire envelope. We discuss the chemistry of the ejecta, and, in particular, the mass fractions of elements that have been studied in the numerous, deep, spectroscopic surveys of globular clusters.

Results. Although oxygen and sodium data are scarce for low-metallicity globular clusters, the small amonut of data avalilable for the unevolved stars in NGC 6397 are compatible with the models. Furthermore, we find good agreement with the C–N anticorrelation of unevolved stars in the cluster M 15. In this cluster, however, no stars of low oxygen ([O/Fe] ~ -1) abundance have been detected. The most massive, very metal-poor clusters, should contain such stars, according to the present models. At the lowest metallicity , the ejecta of the most massive AGBs have C/O > 1, due to the dramatic decrease in the oxygen abundance. We discuss the possible implications of this prediction.