SIMBAD references

We have obtained photometric observations in a 182 arcmin² area, 25' to the north of the center of the globular cluster ω Centauri. The Strömgren vby and broadband BI filters were used to obtain measurements for some 2500 stars. Preliminary examinations of parts of the data have been presented previously (Hughes & Wallerstein, published in 2000; Hughes, Wallerstein, & van Leeuwen, published in 2002). Here we present the complete data set, study the giant branches, and use the B-I color index as a tool for assessing the ages of the populations within ω Cen. Our results confirm previous results for the main-sequence turnoff stars: there appears to be an age spread of about 3-5 Gyr among the stars of ω Cen. We use the proper-motion study by van Leeuwen et al. (published in 2000) to confirm cluster membership above B∼16.5 and calibrate our photometry to that of Hilker and Hilker & Richtler (both published in 2000) in order to use the improved methods described by these authors for deriving metallicity estimates from the Strömgren photometry. We use data on 25 ROA stars in our field and 40 stars from Hilker to look at the enrichment history of ω Cen. We support previous findings that there is another red giant branch, redder (Lee et al., published in 1999, and Pancino et al., published in 2000), and younger than the main giant branch but containing few stars. Even though this so-called RGB-a population appears to be younger than the bulk of the stellar population, it does not seem to be younger than the most metal-rich tail of the bulk population. This last property suggests that the RGB-a may not have been the last burst of star formation in ω Cen and could support the idea, presented by Ferraro et al. and Pancino et al. (both in 2002), that it represents an accreted stellar system.