Astrophys. J., Suppl. Ser., 126, 105-126 (2000/January-0)
A near-infrared photometric study of the low-latitude globular clusters Liller 1, Djorgovski 1, HP 1, and NGC 6528.
Abstract (from CDS):
Images recorded through J,H,Ks,2.2 µm continuum, and 2.3 µm CO filters are used to investigate the stellar contents of the low Galactic latitude 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 the metallicities and reddenings are significantly different from previous estimates. The horizontal branch (HB) in NGC 6528 occurs at K=13.2, and the K luminosity function of HB stars in this cluster is not significantly different from those in the moderately metal-rich clusters NGC 6304 and NGC 6316. R', the ratio of HB to bright red giant branch (RGB) and asymptotic giant 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 HB is consistent with the cluster being very metal rich. The HB 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, and a comparison between the Liller 1 and NGC 6528 sequences on the (K, J-K), and (K,CO) color-magnitude diagrams (CMDs) indicates that Liller 1 has a metallicity comparable to, and perhaps 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 giants, 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 for these clusters are constructed by selecting objects according to CO index. After adjusting for differences in distance and reddening, the Djorgovski 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 clusters are consistent with integrated J-K colors and CO indexes measured through moderately large (∼35"-45") apertures. Based on the brightness of the RGB tip, it is concluded that Djorgovski 1 has a distance modulus µ0=15.4, while the distance modulus of HP 1 is µ0=14.7. The brightest stars in the fields surrounding Liller 1, Djorgovski 1, and HP 1 follow the same relation between MK and CO index as M giants in Baade's window. Stars as faint as the HB have been detected in the field surrounding HP 1, and the relative numbers of HB and upper giant branch stars indicate that R'=1.14±0.11. This relatively low value reinforces earlier studies, which found that R', and hence the helium abundance, likely does not vary with radius in the innermost regions of the Galactic bulge.
globular clusters: individual (Djorgovski 1, HP 1, Liller 1, NGC 6528) - Stars: Late-Type - Infrared: Stars
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