AN ABUNDANCE ANALYSIS FOR FIVE RED HORIZONTAL-BRANCH STARS IN THE EXTREMELY METAL-RICH GLOBULAR CLUSTER NGC 6553

摘要

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/M] 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 #alpha#-process element (Ca) indicates an excess of #alpha#-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.