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

Creators: Carretta, Eugenio; Cohen, Judith G.; Gratton, Raffaele G.; Behr, Bradford B.

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.

Additional Information

© 2001 The American Astronomical Society. Received 2000 March 1; accepted 2001 June 6. 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. We are grateful to Paolo Montegriffo for providing the HST and IRAC photometry of NGC 6528 prior to publication. We warmly thank Bernardo Salasnich for constructing for us the isochrones from the latest Padova models in advance of publication. J. G. C. and B. B. B. are grateful for partial support from NSF grant AST 98-19614. The entire Keck/HIRES user community owes a huge debt to Jerry Nelson, Gerry Smith, Steve Vogt, and many other people who have worked to make the Keck telescope and HIRES a reality and to operate and maintain the Keck Observatory. We are grateful to the W. M. Keck Foundation, and particularly its late president, Howard Keck, for the vision to fund the construction of the W. M. Keck Observatory. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

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