An Abundance Analysis for Four Red Horizontal-Branch Stars in the Extremely Metal-Rich Globular Cluster NGC 6528

Abstract

We present the results of the first analysis of high-dispersion spectra of four red horizontal-branch stars in the metal-rich globular cluster NGC 6528, located in Baade's window. We find that the mean [Fe/H] for NGC 6528 is +0.07+/-0.01 dex (error of the mean), with a star-to-star scatter of σ=0.02 dex (four stars), although the total error is likely to be larger (~0.1 dex) because of systematic errors related to the effective temperature scale and to model atmospheres. This metallicity is somewhat larger than both the mean abundance in the galactic bulge found by McWilliam & Rich (1994) and that found in our previous paper for NGC 6553. However, we find that the spectra of clump stars in NGC 6528 and 6553 are very similar each other, the slightly different metal abundances found being possibly due to the different atmospheric parameters adopted in the two analyses. Since the present analysis is based on higher quality material, we propose to revise our previous published metal abundance for NGC 6553 to [Fe/H]=-0.06+/-0.15. For NGC 6528, we find excesses for the α-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn is found to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O; however, this is somewhat uncertain because of the dependence of the O abundance on the adopted atmospheric parameters and the coupling between C and O abundances in these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~+0.4) in all stars examined. Based in large part on observations obtained at the W. M. Keck Observatory, which is operated operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA). The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.