Collaborative Research: RUI: On the Origin of Abundance Inhomogeneities in Globular Clusters

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In this collaborative program, Dr. Michael Briley (University of Wisconsin - Oshkosh) and Dr. Graeme Smith (University of California - Santa Cruz) will undertake a detailed study of the abundances of various elements in globular cluster stars. It has long been known that stars in an individual cluster all have the same abundance of iron and other heavier elements, but have inhomogeneous abundances in a handful of light elements such as carbon, nitrogen, oxygen, sodium, magnesium, and aluminum. Astronomers now realize that these variations result from an abundance altering process which operates during red giant branch ascent (i.e., some form of deep mixing missing from canonical stellar models) superimposed on primordial variations from the era of star formation. The research team will obtain spectra of CH and CN absorption bands in the violet spectral region, as well as absorption lines from sodium, magnesium, and aluminum at visible-red wavelengths. There are three main goals to the project: to examine the onset and extent of mixing-induced carbon depletion as a function of iron abundance; to measure the fraction of CN-enhanced stars as a function of cluster mass and apparent shape, thereby probing the effects of environment on primordial abundance variations; and to broaden the range of globular clusters in which light element variations have been studied. This project will directly support the training of a graduate student at UCSC and undergraduate students at UW Oshkosh (an undergraduate research institution), who will be directly involved in all aspects of the project. Public outreach and education will be enabled through public talks to schools, clubs, and civic organizations, as well as the development of a presentation for the visitor's gallery of the Lick 3-m telescope that will describe how spectroscopy is used to determine the chemical abundances of stars, and why globular clusters in particular yield an astronomical fossil record of the chemical evolution of galaxies.