STROMGREN PHOTOMETRY OF GALACTIC GLOBULAR CLUSTERS. II. METALLICITY DISTRIBUTION OF RED GIANTS IN ω CENTAURI

We present new intermediate-band Stromgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster omega Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, K-s images). The final optical-NIR catalog covers a region of more than 20 x 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Stromgren metallicity indices, we estimate the photometric metallicity for approximate to 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H](phot) =-1.73 +/- 0.08, -1.29 +/- 0.03, -1.05 +/- 0.02, -0.80 +/- 0.04, -0.42 +/- 0.12, and -0.07 +/- 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail ([Fe/H](phot) less than or similar to -2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP ([Fe/H] <= -1.49), metal-intermediate (MI;-1.49 < [Fe/H] <= -0.93), metal-rich (MR; -0.95 < [Fe/H] <= -0.15), and solar metallicity ([Fe/H] approximate to 0). The last group includes only a small fraction of stars (similar to 8% +/- 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% +/- 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Stromgren metallicity indices display a robust correlation with alpha-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] greater than or similar to -1.7 dex).