IR Photometry of Globular Clusters and Bulge Fields

Abstract

We obtained K vs J-K color-magnitude diagrams (CMDs) for 21 Galactic globular clusters projected onto the bulge. These color-magnitude diagrams reach ~2 magnitudes below the horizontal branch at the distance of the bulge. In particular, the clusters HP1, ES045-SC11, NGC6287, NGC6401, NGC6642, Pal6 and UKS1 have no previously published CMDs. We compared the position of the giant branches with respect to the standard globular clusters M92, M4 and M22, and put all the clusters on a uniform scale. From existing models (Bell, MNRAS, 257, 423,1992; Bessell et al., AAS, 89, 335, 1993) and observations (Frogel, Persson & Cohen, ApJS,53,713, 1983; Brodie & Huchra, ApJ,362,503,1990), we adopt the metallicity calibration [Fe/H] = 5.00( J - K)₀ - 5.0 based on the color of the red giant branch at M _K = -5, valid between [Fe/H] = -2.3 and 0.5. A comparative study is made between the clusters and the bulge stellar population, using photometry in selected fields located a few arcmins from the cluster cores. If the metal abundance, reddening and distance of the globular axe known, one can assume that the reddening is the same, and obtain a mean abundance for the bulge giants. The mean abundance of the bulge RGB for each field is then computed as 〈[Fe/H] _bulge 〉 = [Fe/H] _cluster + 5.0 Δ(J - K). This is essentially the same relation as above, but where the zero point in each field is adjusted to the metallicity of the globular cluster of that field. Hence, these abundances are relative, as is the measure of the color difference between the cluster and the bulge RGBs.