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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Observations of red giants within some globular clusters show star-to-star abundance variations of the elements from C to Al. Since many of these variations depend on luminosity, they must arise during the evolution of these stars. Furthermore, the extent of the variations seems to depend on the cluster metallicity, with the more metal-poor clusters showing large variations in elements up to Al, while the more metal-rich clusters show smaller variations only up to Na.
We explore this discrepancy by following the production of the elements C through Al in realistic red giant branch (RGB) sequences of differing metallicities. This project is designed as a two-step process, one which examines the distribution of these elements around the H-burning shell, followed by one which incorporates the abundance profiles into a mixing algorithm for comparison to the observations. The first step, reported here, combines a detailed nuclear reaction network with the RGB sequences so that we may examine the distribution of elements around the H shell while allowing for the variation of the temperature and density around the shell, and accounting for the change in the structure of the shell with evolution. The reaction rates are varied according their published uncertainties to explore their effects on the final abundance distribution.
We find that we can qualitatively reproduce the the observations of the CNO, NeNa, and MgAl cycle elements in our sequences. As a result of their higher temperatures, longer burning times, and thicker shells, the lower metallicity sequences show variations in all the elements through Al while the higher metallicity sequences exhibit variations only in the CNO and NeNa elements, in accord with the observations. We find, however, that the ^24Mg proton-capture rate must be increased beyond what is presently allowed by the nuclear uncertainties in order to match the observed ^24Mg abundances in M13.
Program listing for Wednesday