V , I CCD photometry of metal-rich bulge globular clusters: NGC 6553

Abstract

We have carried out V I CCD photometry of about 40 000 stars down to V ∼ 23 mag in an area of

about 6' × 10' centered on the metal-rich Galactic bulge globular cluster NGC 6553 and an adjacent field region. Our photometry agrees fairly well with HST data. The cluster population dominates over the field population up to a radial distance of ∼ 3' from the cluster centre. The distance and reddening for the field population present in the direction of the cluster are derived for the first time. These values put the cluster NGC 6553 in the background of the young (age ∼ 800 Myr) Galactic disk stars but

in the foreground of the old Galactic bulge populations. The similar values of E(V − I) ∼ 0.9 for both the disk and the cluster indicate the absence of interstellar matter over a distance of ∼ 3 kpc between the disk and the cluster. An analysis of the giant branch morphology confirms that the metallicity of the cluster population is similar to solar. The red giant branch (RGB) of both cluster and Galactic bulge extend beyond (V − I) = 5 and currently available theoretical isochrones reproduce its shape only

up to (V − I) ∼ 4.5. Our analysis indicates that the presence of differential interstellar extinction across the cluster face causes some elongation and tilt in the HB and produces scatter in the giant branch. The ratio of duration of the RGB-bump phase relative to the life time of the star during the HB phase derived from present observations is in good agreement with the theoretical predictions.