Astron. Astrophys. Suppl. Ser. 135, 391-404

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

R. Sagar^1,2,3 - A. Subramaniam² - T. Richtler²³ - E.K. Grebel⁴⁵

Send offprint request: R. Sagar, Naini Tal

1 - U.P. State Observatory, Manora Peak, Naini Tal, India e-mail: sagar@upso.ernet.in
2 - Indian Institute of Astrophysics, Bangalore 560034, India e-mail: purni@iiap.ernet.in
3 - Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany e-mail: richtler@astro.uni-bonn.de
4 - Astronomisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
5 - University of California/Lick Observatory, Santa Cruz, CA 95064, U.S.A. e-mail: grebel@ucolick.org

Received January 13; accepted October 2, 1998

Abstract:

We have carried out VI CCD photometry of about 40000 stars down to $V\sim$ 23 mag in an area of about $6' \times 10'$ 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 $\sim$ 3$\hbox{$^\prime$}$ 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 $\sim$ 800 Myr) Galactic disk stars but in the foreground of the old Galactic bulge populations. The similar values of $E(V-I)\sim$ 0.9 for both the disk and the cluster indicate the absence of interstellar matter over a distance of $\sim$ 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 $(V-I)\sim$ 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.

Key words: Galaxy: globular clusters: individual: NGC 6553 -- Galaxy: abundances -- stars: late-type -- stars: horizontal branch -- stars: abundances