Deep Infrared Array Photometry of Globular Clusters. V. M28 (NGC 6626)

Details Deep Infrared Array Photometry of Globular Clusters. V. M28 (NGC 6626) Deep Infrared Array Photometry of Globular Clusters. V. M28 (NGC 6626)

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...468..641d&link_type=abstract

Astrophysical Journal v.468, p.641

Astronomy and Astrophysics

Astronomy

17

Infrared: Stars, Galaxy: Globular Clusters: Individual Messier Number: M28, Stars: Evolution, Stars: Hertzsprung-Russell Diagram

Scientific paper

We have obtained B, V, J, and K' images of the heavily obscured, intermediate-metallicity globular cluster M28 (NGC 6626). After reviewing various reddening and metallicity estimates we conclude that E(B-V) = 0.42±0.02 and [Fe/H] = -1.37±0.03. The (V, B-V) color-magnitude diagram (CMD) covers most of the giant branch, and indicates that VHB = 15.5, which is slightly brighter than predicted from earlier photographic photometry. The (K, J-K) CMD extends from the red giant branch (RGB) tip (K 7.6), to below the main-sequence turn-off (MSTO), which occurs at K = 16.5 and J - K = 0.5. The distance modulus estimated from the K brightness of the RGB tip and the V brightness of the horizontal branch is μ0 = 13.4-13.5. The optical and near-infrared CMDs are compared with theoretical isochrones from Bergbusch & VandenBerg (1992) and Straniero & Chieffi (1991). The agreement between theory and observation is not perfect, as the isochrones fall redward of the lower RGB at infrared wavelengths, while the Straniero & Chieffi models do not match the slope of the upper RGB on the (V, B-V) CMD. Nevertheless, the color difference at near-infrared wavelengths between the MSTO and the upper RGB locus is best matched by isochrones with ages near 16 Gyr. We caution that this age estimate should be considered as provisional due to uncertainties in the stellar structure models.
The M28 (K, J-K) CMD is compared with those of M4 (Davidge & Simons 1994) M13 (Davidge & Harris 1995) and M22 (Davidge & Harris 1996). If the M4, M13, and M22 fiducials are shifted to match the M28 MSTO, then the giant branches of all three clusters fall systematically redward of M28. We consider two possible explanations for the relatively blue giant branch of M28. If due to chemical composition, then [O/Fe] in M28 must be near the high end of what is observed among metal-poor stars in the field and clusters. On the other hand, if due to age, then M28 must be ˜4 Gyr older than M13. We suggest observations that will resolve the ambiguity between age and chemical composition; nevertheless, it appears that M28 is at least as old as clusters in the halo, suggesting that the first episodes of star formation in the pro to-Galaxy were not restricted to the halo.

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