The Production of Sodium and Aluminum in Globular Cluster Red Giant Stars

Abstract

We study the production of Na and Al around the hydrogen shell of two red-giant sequences of different metallicity in order to explain the abundance variations seen in globular cluster stars in a mixing scenario. Using detailed stellar models together with an extensive nuclear reaction network, we have calculated the distribution of the various isotopic abundances around the hydrogen shell at numerous points along the red-giant branch. These calculations allow for the variation in both temperature and density in the shell region as well as the timescale of the nuclear processing, as governed by the outward movement of the hydrogen shell. The reaction network uses updated rates over those of Caughlin \& Fowler (1988). We find evidence for the production of Na and Al occurring in the NeNa and MgAl cycles. In particular, Na is significantly enhanced throughout the region above the hydrogen shell. The use of the newer reaction rates causes a substantial increase in the production of $^{27}$Al above the hydrogen shell through heavy leakage from the NeNa cycle and should have an important effect on the predicted surface abundances. We also find that the nuclear processing is considerably more extensive at lower metallicities.