AAS 201st Meeting, January, 2003
Session 108. Old Stellar Populations
Oral, Wednesday, January 8, 2003, 2:00-3:30pm, 618-619

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[108.01] The Giant Branches of Omega Centauri

J.D. Hughes (Everett Community College), G. Wallerstein (University of Washington), F. van Leeuwen (IoA, Cambridge), M. Hilker (University of Bonn)

We have obtained photometric observations in a 182 square arcmin area, 25 arcmin to the north of the center of the globular cluster Omega Cen. The Stromgren vby and broad-band BI filters were used to obtain measurements for some 2500 stars. Our results confirm previous results for the main-sequence turn-off stars: there appears to be an age spread of about 3-5~Gyr among the stars of Omega Cen. We use proper motions to confirm cluster membership above B~ 16.5, and use data on 65 ROA stars to probe the enrichment history of the cluster. We support previous findings that there is another red giant branch, redder and younger than the bulk population of Omega Cen, which contains few stars. Even though this so-called RGB-a population appears to be younger than most 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 Omega Cen, and could support the idea that it represents an accreted stellar system.

The author(s) of this abstract have provided an email address for comments about the abstract: hughes@astro.washington.edu

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