Modelling self-pollution of globular clusters from AGB stars

Abstract

A self-consistent model of the chemical evolution of the globular cluster NGC 6752 is presented to test a popular theory that observed abundance anomalies are due to ``internal pollution'' from intermediate mass asymptotic giant branch stars. We simulated the chemical evolution of the intracluster medium under the assumption that the products of Type II SNe are completely expelled from the globular cluster, whereas material ejected from stars with m < 7 M_sun is retained, due to their weak stellar winds. By tracing the chemical evolution of the intracluster gas, we tested an internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. The observed spread in Na and Al was reproduced, but not the O-Na and Mg-Al anticorrelations. In particular, neither O nor Mg are sufficiently depleted to account for the observations. We predict that the Mg content of Na-rich cluster stars should be overwhelmingly dominated by the 25,26Mg isotopes, whereas the latest data shows only a mild 26Mg enhancement and no correlation with 25Mg. Furthermore, stars bearing the imprint of intermediate mass stellar ejecta are predicted to be strongly enhanced in both C and N, in conflict with the empirical data. We find that while standard AGB stellar models do show the hot H burning that seems required to explain the observations, this is accompanied by He burning, producing primary C, N, Mg and Na (via HBB) which do not match the observations. (Abridged)