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Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
We have used the 3.6 metre Canada-France-Hawaii Telescope to obtain B, V, J, and K images of selected fields in the globular cluster M28. This cluster is of interest for studies of the early evolution of the Galaxy because it is moderately metal-poor ([Fe/H] = -1.3) and has a Galacto-centric distance of only 3 kpc. The B and V data are restricted to the cluster centre, and the (V, B-V) CMD shows a well-defined giant branch, the shape and colour of which are consistent with existing metallicity and reddening estimates. The J and K observations cover three fields: the cluster centre; a region 3 arcmin from the centre; and a background field. The composite (K, J-K) CMD extends almost 1 mag fainter than the main sequence turn-off, and hence can be used to investigate the age of M28. Comparisons with theoretical isochrones suggest that M28 is very old, with an age in the range 16 -- 20 Gyr. Moreover, the difference in J-K colour between the main sequence turn-off and base of the giant branch, a statistic that is sensitive to age, is smaller in M28 than in the halo cluster M13. While this result is suggestive of a significant difference in age, in the sense that M28 is older than M13, it could also be indicative of a difference in chemical composition, such that [O/Fe] is higher in M28 than in M13.
Program listing for Wednesday