Stellar abundances have been critical to understanding the formation and chemical evolution of the halo, disk, and bulge of the Galaxy. With the advent of infrared imaging and spectroscopy on large telescopes, it is now possible to explore the stellar population within 100 pc of the Galactic center in detail. The Galactic center contains many young, luminous, massive stars, which are concentrated in three separate clusters within the central 60 parsecs of the Galaxy. These are the Central Cluster, the Arches Cluster, and the Quintuplet Cluster, with respective ages for the youngest stars of 3-9 Myr, 1-4.5 Myr, and 3-4 Myr. The Central Cluster, located at the dynamical center of the Galaxy, has a striking excess of young, luminous stars compared to the bulge stellar population in Baade's window. These young stars co-exist in the Central Cluster with older stars whose ages range up to a few 100 Myr or more. The relatively young ages (1 - 100 Myr) of the brightest stars in the Galactic center support the idea that the Galaxy's central bar has driven disk gas into the Galactic center to fuel star formation throughout the Galaxy's history, continuously or in bursts of star formation.
Previous work by Dr. Kristen Sellgren, at Ohio State University, on differential stellar abundance measurements of [Fe/H] within the Central Cluster and the Quintuplet Cluster have found that the distribution and mean value of stellar [Fe/H] in the central 60 parsec of the Galaxy is indistinguishable from the values they derive for stars of similar temperature and luminosity in the solar neighborhood. This result is surprising, because gas-phase abundance measurements of H II regions show a significant increase in the alpha-elements O and S between the solar neighborhood and the Galactic center. One explanation for this is that the alpha-capture elements compared to Fe, [alpha/Fe], are enhanced in the Galactic center. The star formation conditions in the Galactic center are very different from those in the solar neighborhood, and are predicted to result in an initial mass function (IMF) weighted toward massive stars. Furthermore, models of supernova enrichment in an environment dominated by massive stars predict a high value of [alpha/Fe]. The current project directed by Dr. Sellgren will test this hypothesis by obtaining spectra to determine [alpha/H] in Galactic center stars for which [Fe/H] has been already derived from previous work. A key part of this study will be to perform a differential analysis of Galactic center stars compared to solar-neighborhood stars with a similar range in effective temperature and surface gravity. The observation and analysis of comparison stars in the solar neighborhood is critically important in Galactic center abundance studies in order to reduce the effects of systematic errors. The abundance analysis itself involves the computation of the synthetic spectrum of each star, using the spectral synthesis program MOOG and model atmospheres for late type giants and supergiants. This project expects to obtain spectra and derive CNO abundances of more solar neighborhood M supergiants and more Galactic Center M supergiants to determine whether the anomalous CNO abundances in the source IRS 7 are due to the unique star-forming environment in the Galactic Center compared to the solar neighborhood, the presence of the black hole in the Central Cluster, or the initial CNO abundances from which solar neighborhood and Galactic center stars formed. The study of alpha-elements in a sample of Galactic center stars will answer fundamental questions about the nature of star formation, gas infall and outflow, and chemical evolution in the nucleus of the Galaxy. ***
