Deep HST Photometry of NGC 6388: Age and Horizontal-Branch Luminosity
Kinemuchi, Karen; Rich, R. M.; Catelan, M.; Stetson, Peter B.; Pritzl, Barton J.; Smith, Horace A.; Layden, Andrew C.; Sweigart, Allen V.
Chile, Canada, United Kingdom, United States
Abstract
Using the Hubble Space Telescope, we have obtained the first color-magnitude diagram (CMD) to reach the main-sequence turnoff of the Galactic globular cluster NGC 6388. From a comparison between the cluster CMD and that of 47 Tucanae, we find that the bulk of the stars in these two clusters have nearly the same age and chemical composition. On the other hand, our results indicate that the blue horizontal branch and RR Lyrae components in NGC 6388 are intrinsically overluminous, which must be due to one or more, still undetermined, noncanonical second parameter(s) affecting a relatively minor fraction of the stars in NGC 6388.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555, and on observations retrieved from the ESO ST-ECF Archive.