Proton-Capture Nucleosynthesis in Globular Cluster Red Giant Stars

Abstract

We follow the static abundance profiles of the C, N, O, Ne, Na, Mg, and Al isotopes around the hydrogen-burning shell (H shell) of globular cluster red-giant-branch (RGB) stellar models in order to study the observed abundance anomolies associated with these elements. We do this by combining four "realistic" stellar evolutionary sequences of different metallicities with a detailed nuclear reaction network. This approach allows for the variation in the temperature and density around the H shell as well as structural changes in the H shell caused by advancing evolution and variations in metallicity. In addition, our reaction network allows us to explore the effects of the uncertainties in the reaction rates on the abundance profiles. Our results are qualitatively consistent with the observed C vs. N, and O vs. N, Na, and Al anticorrelations as well as with the observed luminosity- and metallicity-dependent variations in the abundances and isotopic ratios. We find that the source of the large Al enhancements in some clusters is 24Mg, but contrary to the results of Shetrone (1996b;1997) we are unable to deplete the 24Mg abundance due to leakage from the NeNa cycle via the 23Na(p,gamma)24Mg reaction. We discuss possible remedies to this discrepancy.