On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test

Abstract

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globular clusters. We have tested, with fully consistent stellar evolution calculations, whether theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the color-magnitude diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations and varying initial He mass fractions. Comparisons with ``normal'' α-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the red giant branch tip are also very weakly affected, apart from-depending on the He content of the polluting material-the red giant branch bump region. We also study the distribution of stars along the zero-age horizontal branch and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.