Contributions from Stellar Mixing and Primordial Enrichment to Globular Cluster Abundances

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Smith, Graeme" Contributions from Stellar Mixing and Primordial Enrichment to Globular Cluster Abundances "AST- 0098453Globular clusters are among the oldest objects that we recognize. Determinations of their ages are critical for setting limits to the age of the universe. In addition, because of their long lifetimes, they contain within their constituent stars a "fossil record" of the chemical history of our own galaxy. However, the situation is more complicated that one might hope. Although globular clusters were once thought to be chemically homogeneous objects, spectroscopy has shown that the brightest red giant stars within the same clusters commonly exhibit abundance differences in the surface elements C-N-O-Na-Mg-Al. At least some of these differences appear to arise from deep mixing in which material is transported from near the hydrogen-burning shell to the surface of the star. But over and above this, there are the primordial abundances which were set when some cluster stars were formed from gas which had been selectively enriched in heavy elements from high-mass stars of a former epoch. The present award looks to a main goal of measuring abundances of red giants in clusters that are less metal poor than those previously studied. This will allow the investigation of the effects of metallicity on the deep mixing process. Deep mixing as a function of stellar mass will also be investigated. The degree to which primordial effects have contributed to globular cluster abundance inhomogeneities will be investigated by spectroscopy of the fainter red giant stars and main sequence stars in several other globular clusters. Here one expects that deep mixing has been an effective agent. Observations with the Keck telescopes and associated spectrographs form the core of this work. Interpretation will follow from the application of state-of-the-art model atmosphere theory.