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.
