V, I CCD photometry of metal-rich globular clusters: NGC 6528

Abstract

We present CCD photometry in V and I for the metal-rich globular cluster NGC 6528. A comparison with previous photometry reveals discrepancies of the order 0.1 to 0.2 mag in V, emphasizing the need for independent photometry. As found previously, the giant branch (or the asymptotic giant branch, which cannot be distinguished) of the cluster extends to V-I = 7. Population synthesis in V-I must take these red stars into account in order to understand integrated red colours of bulge populations. Currently available theoretical isochrones do not reproduce the shape of the RGB/AGB. The derivation of reliable values for reddening, distance and metallicity of NGC 6528 is hampered by uncertainties in the extrapolation to high metallicity, which dominate the absolute error rather than the photometric uncertainty does. Together with other properties (old age derived from HST data, high radial velocity), this object is more likely a member of the bulge than of the disk population.