Strömgren photometry of the Bulge: the Baade’s Window and the globular cluster NGC 6522

We present Ca−uvby Strömgren photometry of the Baade’s Window, including the Galactic globular cluster NGC 6522. We separate field and cluster stars by adopting color–color planes and proper motions. We then estimate the global metallicity of red-giants (RGs) in NGC 6522 by using a new theoretical metallicity calibration of the Strömgren index hk presented in (Calamida et al. 2011). We find that metallicities estimated by adopting the hk index and the Ca–y color are systematically more metal-rich than metallicities estimated with hk and the u–y, v–y and b–y colors. Current evidence indicate that the hk metallicity index is affected not only by the Ca abundance, but also by another source of opacity. 1 Introduction and data reduction The study of the Milky Way bulge is fundamental to understand the star formation history of the Galactic spheroid. The Milky Way bulge is indeed dominated by metal-rich old stars (t> 10 Gyr, Zoccali et al. 2003). Their metallicity distribution is centered on solar metallicity and span a range from [Fe/H] ∼-1.5 to 0.5 dex, based on the high-resolution spectroscopy o