First Name:
Jeffrey
Last Name:
Gerber
Major Department:
Physics
Thesis Director:
Michael Briley
Date of Thesis:
May 2014
It was discovered some thirty years ago that stars in Galactic globular clusters tend to decrease in carbon abundance with increasing luminosity on the red giant branch, particularly among the lower metallicity clusters. While such a phenomena is not predicted by canonical models of stellar interiors and evolution, it is widely believed to be the result of some extra mixing operating during red giant branch ascent which transports material exposed to the CN(O)-cycle to the surface. Here we present an analysis of observations in the evolving red giants of globular clusters within our own Galaxy. Building on the work of Martell, Smith, and Briley (2008, AJ, 136, 2522), we have used the KPNO 4-m and SOAR 4.1-m telescopes to extend the sample of clusters. The CH absorption features in these low resolution blue spectra have been analyzed via synthetic spectra in order to obtain [C/Fe] abundances. These abundances and the luminosities of the target stars were used to establish the rate at which C abundances are changing with time (i.e., the mixing efficiency). By establishing rates over a wide range of composition, the dependence of deep mixing on metallicity can be determined and used to better constrain theories of the underlying process. We find that not only do our carbon rates decrease as a function of metallicity as expected, but the carbon rates are heavily dependent on the initial [C/Fe] composition of the star.\footnote{This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.}
Complete Thesis (PDF, 245KB)