Photometric Observations of ω Centauri: Multi-Wavelength Observations of Evolved Stars

We present multi-wavelength observations of the northern population of ω Cen from the main-sequence turn-off to high on the red giant branch. We show that the best information about the metallicity and age of the stars can be gained from combining vby, B-I and V-I colors (in the absence of spectroscopy). We confirm our results for the main-sequence turn-off stars: there is at least a 3 Gyr age spread, which may be as large as 5-8 Gyr, as suggested by Hughes & Wallerstein (2000) and Hilker & Richtler (2000). We find that B-I colors can be affected by excessive CN-content (which can vary at a given value of [Fe/H]). We use proper motion studies to confirm cluster membership at and above the level of the horizontal branch, and we show that the age spread is maintained amongst stars from the subgiant branch through the red giants. We support previous findings that there is another red giant branch, redder (Pancino et al. 2000), and younger than the main giant branch (Hilker & Richtler 2000) but containing few stars. Even though the spatial distribution of the metal-rich stellar population is different from that of the metal-poor population (which could be achieved by capturing a smaller metal-rich star cluster, as suggested by Pancino et al. 2000), it is likely that ω Cen is the core of a captured dwarf galaxy.