Other
Scientific paper
Jun 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009phdt.........7j&link_type=abstract
Proquest Dissertations And Theses 2009. Section 0093, Part 0606 469 pages; [Ph.D. dissertation].United States -- Indiana: India
Other
Chemical Abundances, Galaxy Evolution, Milky Way Galaxy, Open Clusters, Stars, Abundance Transition Zone
Scientific paper
Detailed chemical abundance studies of open star clusters in the Milky Way Galaxy provide crucial information about the formation and evolution of our Galaxy and of other disk galaxies in general. Numerous studies have shown that the open cluster metallicity distribution decreases with increasing distance from the center of the Galaxy, in agreement with studies of other disk populations. Some recent studies in the last decade have uncovered evidence that the metallicity distribution may not be a linear gradient as originally thought, but rather a step function with a sharp break in the abundance distribution at a Galactocentric radius of R gc 10-12 kiloparsecs (kpc). In this dissertation, I examine the nature of the transition from the inner disk to the outer disk of our Galaxy by determining detailed chemical abundances for nearly twenty open clusters evenly distributed throughout the supposed transition region. My sample more than doubles the number of clusters previously studied in this part of the Galaxy. Chemical abundances for several individual stars in each cluster are determined from high-resolution spectroscopy obtained with four different telescope/instrument combinations, including the Hydra multi-object spectrograph on the WIYN 3.5m telescope. Temperatures and surface gravities of individual stars in each cluster are determined using these spectra and available photometry, and stellar chemical abundances are averaged together to form a robust cluster mean abundance and dispersion. Cluster distances are also placed on a homogeneous scale. For analysis of the open cluster abundance distribution across the full extent of the Milky Way disk, my cluster sample is combined with two other samples in the literature. Through visual examination of the resulting cluster abundance distributions as a function of Galactocentric distance and the use of a statistical mixture modeling algorithm, it is found that the chemical abundance distribution in the region R gc ~10-12 kpc is well represented by a gradient, and is inconsistent with a sharp transition at any Galactocentric distance. These findings are consistent with the abundance distributions seen in other disk galaxies, and can be reproduced by theoretical models of galaxy formation and chemical evolution.
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