An Abundance Analysis for Five Red Horizontal-Branch Stars in the Extremely Metal-rich Globular Cluster NGC 6553
Cohen, Judith G.; Carretta, Eugenio; Gratton, Raffaele G.; Behr, Bradford B.
United States, Italy
Abstract
We provide a high-dispersion line-by-line abundance analysis of five red horizontal-branch (HB) stars in the extremely metal-rich Galactic globular cluster NGC 6553. These red HB stars are significantly hotter than the very cool stars near the tip of the giant branch in such a metal-rich globular cluster, and hence their spectra are much more amenable to an abundance analysis than would be the case for red giants. We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the mean abundance in the Galactic bulge found by McWilliam & Rich and considerably higher than that obtained from an analysis of two red giants in this cluster by Barbuy and coworkers. The relative abundance for the best-determined α-process element (Ca) indicates an excess of α-process elements of about a factor of 2. The metallicity of NGC 6553 reaches the average of the Galactic bulge and of the solar neighborhood.
Based in large part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California.