IR color-magnitude diagrams of 20 galactic globular clusters and bulge fields

We present K vs J-K color-magnitude diagrams for 20 Galactic globular clusters projected onto the bulge. These color-magnitude diagrams reach ∼2 mag below the horizontal branch at the distance of the bulge. Some of these clusters (HP 1, ESO45-SC11, NGC 6284, NGC 6401, NGC 6642, Pal 6, and UKS 1) have no previously published CMDs. The candidate clusters TJ 5, TJ 15, and TJ 16 are not detected in our deep IR images, it is likely that these are not real globulars. We compare 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 using the best available reddenings and metallicities. Reddenings are derived for the less studied clusters, based on the position of the red giant branches in the near-IR color-magnitude diagrams. A comparative study is made between the clusters and the bulge stellar population, using photometry in selected fields located a few arcminutes away from the cluster cores. Our major conclusion is that the bulge giants have the same near-IR colors in the mean as the most metal-rich globulars. From existing models and observations, we adopt the calibration law [Fe/H]=5.0(J-K)-5.60 based on the color of the red giant branch at K₀=-5.5, valid between [Fe/H]=-2.3 and 0.5. On the basis of differential colors of the cluster giant branch against the field giant branch, we find a metallicity gradient in the bulge fields that supports previously published results from optical photometry.