CCD Photometry of the Globular Cluster Centauri. II. Stellar Populations and Age-Metallicity Relation

We present wide-field and high-precision BV and Ca & Strömgren by photometry of ω Centauri, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of ω Cen are well discriminated from foreground Galactic field stars in the hk [=(Ca −b) −(b −y)] vs. b − y diagram. The resulting “cleaned ” color-magnitude diagram (CMD) has allowed us to obtain an accurate distribution of the red horizontal branch (HB) and the asymptotic giant branch stars. We confirm the presence of several red giant branches (RGBs) with the most metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest that four populations with different metallicities can reproduce the observed nature of the RGB. The HB distribution is also found to be consistent with the multiple stellar populations of the RGB. From our population models, we propose that 1