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Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[82.05] Deep IR and Optical Photometry of the Bulge Globular Cluster M28 (NGC6626)

T. J. Davidge (Gemini Canada), W. E. Harris (McMaster), P. Côté (DAO)

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