A near-infrared photometric study of the low-latitude globular clusters Liller 1, Djorgovski 1, HP 1, and NGC 6528

Images recorded through J, H, Ks, 2.2 mu m continuum, and 2.3 mu m CO filte rs are used to investigate the stellar contents of the low Galactic latitud e globular clusters NGC 6528, Liller 1, Djorgovski 1, and HP 1, as well as surrounding bulge fields. Metallicities are estimated for the latter three clusters by comparing the colors and CO indexes of giant branch stars with those in other clusters and the bulge, while reddenings are estimated from the colors of bright bulge stars in the surrounding fields. In some cases t he metallicities and reddenings are significantly different from previous e stimates. The horizontal branch (HB) in NGC 6528 occurs at K = 13.2, and th e K luminosity function of HE stars in this cluster is not significantly di fferent from those in the moderately metal-rich clusters NGC 6304 and NGC 6 316. R', the ratio of HE to bright red giant branch (RGB) and asymptotic gi ant branch (AGB) stars, is 1.25 +/- 0.25, in good agreement with its value in other globular clusters. The RGB bump in NGC 6528 is detected at K = 13. 8, and the relative brightness of this feature with respect to the HE is co nsistent with the cluster being very metal rich. The HE in Liller 1 occurs at K = 14.4. Stars in Liller 1 have smaller J-K colors and CO indexes than bulge giants with the same brightness, acid a comparison between the Liller 1 and NGC 6528 sequences on the (K, J-K), and (K, CO) color-magnitude diag rams (CMDs) indicates that Liller 1 has a metallicity comparable to, and pe rhaps even slightly lower than, that of NGC 6528. Stars in Liller 1 and NGC 6528 occupy the same region of the (J-H, H-K) two-color diagram as bulge g iants, which is consistent with these clusters forming as part of the bulge . Stars in Djorgovski 1 and HP 1 have significantly smaller CO indexes than stars with the same brightness in the surrounding bulge fields, and CMDs f or these clusters are constructed by selecting objects according to CO inde x. After adjusting for differences in distance and reddening, the Djorgovsk i 1 giant branch on the (K, J-K) CMD is well matched by that of the [Fe/H] = -2.2 cluster M92, while the giant branch of HP 1 follows that of the [Fe/ H] = -1.6 globular cluster M13. The metallicities inferred for both cluster s are consistent with integrated J-K colors and CO indexes measured through moderately large (similar to 35 "-45 ") apertures. Based on the brightness of the RGB tip, it is concluded that Djorgovski 1 has a distance modulus m u(0) = 15.4, while the distance modulus of HP 1 is mu(0) = 14.7. The bright est stars in the fields surrounding Liller 1, Djorgovski 1, and HP 1 follow the same relation between M-K and CO index as M giants in Baade's window. Stars as faint as the HE have been detected in the held surrounding HP 1, a nd the relative numbers of HE and upper giant branch stars indicate that R' = 1.14 +/- 0.11. This relatively low value reinforces earlier studies, whi ch found that R', and hence the helium abundance, likely does not vary with radius in the innermost regions of the Galactic bulge.