Infrared array photometry of metal-rich globular clusters .3. Two more clusters and an analysis of V-K colors

We present new JHK photometry for the disk globular clusters NGC 6440 and NGC 6624. These data are initially used to confirm and refine several important results from Kuchinski et al. [AJ, 109, 1131 (1995)] for other disk globulars. First, we again demonstrate the ability to derive a reddening-independent estimate for the [Fe/H] of a cluster from the slope if its giant branch (GB) in a K, J-K color-magnitude diagram (CMD). Second, the reddening corrected J-K color and K magnitude of the center of the horizontal branch (HE) and the J-K color of its red edge are confirmed to be independent of [Fe/H] fur these clusters. Thus these parameters can be used to estimate E(J-K) of metal-rich clusters with no knowledge of distance or [Fe/H] and to estimate (m-M) if one can first estimate the reddening. We also confirm that the reddening-independent quantities, the half width of a cluster's horizontal branch (HB), and the color difference between the center of the HE and the GB at the level of the HE, both appear to be insensitive to metallicity. The JHK colors of NGC 6440 are similar to those of Liller 1, in both cases these colors are unlike those seen for other globular clusters, field giants, or bulge giants. We have not been able to identify any other cluster parameter that would help to explain these anomalous colors. We have assembled V photometry from the literature for the clusters in our sample and VK photometry for two additional disk globular clusters from Davidge et al. [ApJS, 81, 251 (1992)]. We conclude that K, J-K CMDs are preferable to K, V-K CMDs as tools to study basic cluster properties. Finally, we compare our data with theoretical isochrones for metal-rich clusters and present observational evidence that the dependence of the V-K color of the GB on [Fe/H] may be different for halo and disk globular clusters. This difference may be related to differences in the [O/Fe] values for the two cluster systems. (C) 1995 American Astronomical Society.