INFRARED ARRAY PHOTOMETRY OF METAL-RICH GLOBULAR-CLUSTERS .1. TECHNIQUES AND FIRST RESULTS

As a first step in investigating the properties of heavily reddened, metal-rich globular clusters, we present JHK colors and magnitudes for four objects: NGC 5927, NGC 6712, NGC 6838 (M71), and Terzan 2. These data begin near the top of the giant branch (GB) and extend to stars six magnitudes fainter, reaching below the horizontal branch (HB). We use our sample and previously published data for two other clusters, 47 Tucanae and M69, to develop techniques for deriving basic properties such as metallicity and distance modulus from observable parameters of cluster color-magnitude diagrams (CMDs) in the near-IR. We also analyze the horizontal branches of the clusters in an attempt to set constraints on mass loss for first-ascent giants. We find that the slope of the linear fit to the GB stars brighter than the HB can be used to predict metallicity to an accuracy of ±0.25 dex for clusters with [Fe/H] between -1.0 and -0.3. There appears to be no significant dependence on [Fe/H] for other reddening-free indices measured from the CMD, including the width of the HB and the separation between the GB and HB at the level of the HB. Using the clusters in our sample that have known distances, we derive a value of 〈MK0(HB)〉=-1.15±0.10 for the extinction-corrected absolute K magnitude of the HB. This provides a means of determining the distance to clusters for which only infrared CMDs are available. We derive a distance modulus to Terzan 2 of 14.37±0.2 which places it within a few hundred parsecs of the galactic center. In the J-H, H-K diagram, Terzan 2 giants are located near the Baade's Window giants rather than in the region populated by nearly all other globular cluster stars. The GB of stars in the field around Terzan 2 has a slope that implies [Fe/H]=+0.1, comparable to that in Baade's Window.