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Scientific paper
Jun 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........18m&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0036, Part 0606 176 pages; [Ph.D. dissertation].United States -- California: Un
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Globular Clusters, Elemental Abundance, Stars, Red Giants
Scientific paper
This dissertation describes investigations into two of the persistent questions of elemental abundances in Galactic globular clusters: the phenomenon of deep mixing, observed through the progressive depletion of surface carbon abundance as stars evolve along the red giant branch, and abundance bimodality, a phenomenon observed only in globular clusters, in which a subset of stars in a given globular cluster have a distinctive pattern of elemental enhancements and depletions relative to the Solar pattern.
The first chapter gives an introduction to the history of globular cluster abundance studies, with particular focus on low-resolution spectroscopy. For both deep mixing and abundance bimodality, the leading theoretical models and the data which support and challenge them are laid out. Each section ends with a description of presently-unanswered questions; these are the motivation for the various projects contained in this dissertation.
The second chapter describes the use of molecular handstrengths for determining elemental abundances from low-resolution spectra, and introduces a new CH bandstrength index that is designed to be sensitive to carbon abundance and insensitive to nitrogen abundance in Pop. II red giants over a wide range of metallicity. Various CH indices defined elsewhere in the literature are also discussed, and are shown to have comparable accuracy to the new index only over a limited range of stellar properties. Carbon abundances determined using the new CH index are compared to literature abundances for a few stars, and general concordance with published abundances is found.
The third chapter contains a large-scale application of the new CH index: a survey of present-day carbon abundances and calculated carbon depletion rates in bright red giants belonging to eleven Galactic globular clusters spanning the full metallicity range of halo globular clusters. Targets were selected with similar evolutionary states, were observed with one instrument on one telescope, and were analyzed in a uniform manner, to eliminate potential sources of significant systematic error. In keeping with present theoretical models of the deep mixing process, red giants in low-metallicity globular clusters undergo more rapid carbon depletion, and therefore more efficient deep mixing, than their high-metallicity counterparts.
The fourth chapter investigates the apparent disappearance of abundance bimodality at low overall metallicity by determining carbon abundances along the full red giant branch of the globular cluster M53. I find that the mild variation of CN bandstrength observed is consistent with a mildly bimodal distribution in carbon abundance, in agreement with previous work on the subject. However, this result raises the question of whether all low- metallicity clusters should have only mild star-to-star abundance variations, or whether M53 is unusual in this regard. I discuss previous investigations into this same question using other low-metallicity globular clusters, and conclude that M53 may have milder abundance variations than the well-studied clusters M92 and M15.
The fifth chapter describes a study of CN bandstrength behavior in high- metallicity star clusters. While abundance bimodality, observed from a bimodal CN bandstrength distribution and anticorrelated CN and CH bandstrengths, is universal among Galactic globular clusters, it is not observed in Galactic open clusters. It is also unobserved among stars in the general halo field, an observation which places strong constraints on the process which produces the abundance bimodality. I find that the high-metallicity disk globular clusters NGC 6356 and NGC 6528 show clear CN bimodality, indicating that they are more similar to other (low-metallicity) globular clusters than to the old open clusters NGC 188, NGC 2158, and NGC 7789.
The final chapter summarizes the questions addressed and the conclusions reached in the various projects described in this dissertation. Possible future work, which would help to clarify or extend those results, is also briefly outlined.
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