Abstract

We present Stromgren and near-infrared (NIR) photometry of the bulge cluster NGC 6528 and its surrounding field in Baades Window. uvby images were collected with EFOSC2 on the New Technology Telescope (NTT, La Silla, ESO). The NIR catalogs are based on J,K-band VIRCAM at VISTA (Paranal, ESO) and SOFI at NTT photometry. We matched the aforementioned data sets with Hubble Space Telescope photometry to obtain proper-motion-cleaned samples of NGC 6528 and bulge stars. Furthermore, we were able to correct the Stromgren-NIR photometry for differential reddening. The huge color sensitivity of the Stromgren-NIR colormagnitude-diagrams (CMDs) helped us in separating age and metallicity effects. The red giant branch (RGB) of NGC 6528 is well reproduced in all the CMDs by adopting scaled solar isochrones with solar abundance, that is Z = 0.0198, or alpha-enhanced isochrones with the same iron content, that is Z = 0.04, and an age range of t = 1012 Gyr. The same isochrones well reproduce most of the color spread of Baades Window RGB. These findings support the literature age estimates for NGC 6528. We also performed a new theoretical visual-NIR metallicity calibration based on the Stromgren index m(1) and on visualNIR colors for red giant (RG) stars. Scaled solar and alpha-enhanced models were adopted and we validated the new metallicity-index-color (MIC) relations by applying them to estimate the photometric metal abundance of a sample of field RGs and of a metal-poor (M 92, [Fe/H]similar to-2.3) and a metal-rich (NGC 6624, [Fe/H] similar to -0.7) globular cluster. We applied the calibration to estimate the mean metal abundance of NGC 6528, finding [Fe/H] = [M/H] = -0.04 +/- 0.02, with a mean intrinsic dispersion of sigma = 0.27 dex, by averaging the metallicities obtained with the scaled solar [m], y - J and [m], y - K MIC relations, and of -0.11 +/- 0.01, with sigma = 0.27 dex, by using the m1, y - J and m1, y - K relations. These findings support results based on high-resolution spectroscopy, which approximately give a solar metallicity for NGC 6528 with a modest ?-enhancement. By applying the scaled solar MIC relations to the sample of Baades Window RGs, we find a metallicity distribution that extends approximately from [Fe/H] similar to -1.0 up to [Fe/H] approximate to 1 dex, with two metallicity peaks at [Fe/H] approximate to -0.2 and [Fe/H] approximate to 0.55 ([m] , y - J and [m] , y - K relations), and [Fe/H] approximate to -0.25 and [Fe/H] approximate to 0.4 (m1, y - J and m1, y - K relations). These findings fairly agree with previous spectroscopic studies for Baades Window and the recent results of the ARGOS surveys for the bulge.