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

We study the production of Na and Al around the hydrogen shell of twored-giant sequences of different metallicity in order to explain the abundancevariations seen in globular cluster stars in a mixing scenario. Using detailedstellar models together with an extensive nuclear reaction network, we havecalculated the distribution of the various isotopic abundances around thehydrogen shell at numerous points along the red-giant branch. Thesecalculations allow for the variation in both temperature and density in theshell region as well as the timescale of the nuclear processing, as governed bythe outward movement of the hydrogen shell. The reaction network uses updatedrates over those of Caughlin \& Fowler (1988). We find evidence for theproduction 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 theproduction of $^{27}$Al above the hydrogen shell through heavy leakage from theNeNa cycle and should have an important effect on the predicted surfaceabundances. We also find that the nuclear processing is considerably moreextensive at lower metallicities.Comment: 4 pages with 4 EPS figures embedded, accepted by ApJL March 28, 199