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

We present the results of the first analysis of high dispersion spectra offour red HB stars in the metal rich globular cluster NGC 6528, located inBaade'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 sigma = 0.02 dex (4 stars),although the total error is likely to be larger (~0.1 dex) due to systematicerrors related to the effective temperature scale and to model atmospheres.This metallicity is somewhat larger than both the mean abundance in thegalactic bulge found by McWilliam & Rich (1994) and that found in our previouspaper for NGC 6553. However, we find that the spectra of clump stars in NGC6528 and NGC 6553 are very similar each other, the slightly different metalabundances found being possibly due to the different atmospheric parametersadopted in the two analyses. For NGC 6528 we find excesses for thealpha-process elements Si and Ca ([Si/Fe]=+0.4 and [Ca/Fe]=+0.2), whereas Mn isfound to be underabundant ([Mn/Fe]=-0.4). We find a solar abundance of O;however this is somewhat uncertain due to the dependence of the O abundance onthe adopted atmospheric parameters and to coupling between C and O abundancesin these cool, metal-rich stars. Finally, we find large Na excesses ([Na/Fe]~+0.4) in all stars examined. Since the present analysis is based on higherquality material, we propose to revise our previous published metal abundancefor NGC 6553 to [Fe/H]=$0.06+-0.15.Comment: accepted for publication in the September 2001 issue of The Astronomical Journal; 3 new figures and updated results and calibration