Deep near-IR photometry of eight metal-poor globular clusters in the Galactic bulge and halo

Abstract

High quality J, H and K images are used to investigate the morphological properties of the near-infrared color-magnitude diagrams for five metal-poor bulge globular clusters and three halo clusters. Photometric parameters to describe the RGB shape, i.e., the colors at fixed magnitudes of M K = M H = (−5.5, −5, −4 and −3), the magnitudes at fixed colors of (J − K) o = (J − H) o = 0.70, and the RGB slope, have been measured from the fiducial normal points of the CMDs. We also measured the near-infrared magnitudes of the RGB bump and tip on the luminosity function of the RGB stars for each cluster. The RGB parameters of the observed metal-poor bulge and halo clusters are consistent with the previous empirical relationships between the RGB parameters and the cluster metallicity for metal-rich bulge clusters and halo clusters. The near-infrared magnitudes of the RGB bump and tip are in good agreement with the theoretical prediction of the Yonsei-Yale isochrone.

Near-infrared properties of metal-poor globular clusters in the Galactic bulge direction

“…The photometric calibration equations obtained from UKIRT standard stars were then applied to the magnitudes of the stars on the target clusters. Standardizations were also double-checked in direct star-to-star comparison with the photometry of bright stars in M13 of Kim et al (2006). As shown in Kim et al (2006), there are only small photometric offsets, K = 0.03 ± 0.01 and (J − K) = 0.04 ± 0.01, between the photometric data with the UKIRT system and those with the 2MASS system of Valenti et al (2004c).…”

“…Standardizations were also double-checked in direct star-to-star comparison with the photometry of bright stars in M13 of Kim et al (2006). As shown in Kim et al (2006), there are only small photometric offsets, K = 0.03 ± 0.01 and (J − K) = 0.04 ± 0.01, between the photometric data with the UKIRT system and those with the 2MASS system of Valenti et al (2004c). The offsets will become negligible after the transformation of the fiducial normal points for the observed near-infrared CMD into the 2MASS photometric system (see Sect.…”

“…Scattering in the near-infrared CMDs of the target clusters might be due to both photometric errors and contamination by foreground field stars toward the Galactic bulge. It is apparent that the highly reddened tiny cluser Terzan 4 with the tidal radius 7.6 ± 1.3 and To examine the relationship between the RGB morphological parameters in CMDs of the absolute plane and cluster's metallicity, the values of metallicity, reddening, and distance modulus were estimated for each cluster using the method adopted in Kim et al (2006). Metallicities for target clusters are used in the Carreta & Gratton (1997) scale, [Fe/H] CG97 , to directly compare the photometric properties of the measured RGB morphology with the results presented in Valenti et al (2004aValenti et al ( , 2007.…”

“…Synthetic HBs for each cluster with different metallicities were generated by the method used in Lee et al (1994) with the HB evolutionary tracks of Yi et al (1997). The details of the generated synthetic HBs with various metallicities are described in Kim et al (2006). The synthetic HBs in the absolute plane were transformed into the observed HBs in the CMDs of the target clusters from Rich et al (1998) for NGC 6558, Ortolani et al (1997) for Terzan 4, and Piotto et al (2002) for the eight other clusters.…”

“…In addition, high extinctions toward the bulge can be reduced by observing the near-infrared wavelengths, as the extinction in the K band is only ∼10 percent of that in the V band (Rieke & Lebofsky 1985). Using these bases, Kim et al (2006) presented the morphological properties of the RGB in the near-infrared CMDs for five metal-poor clusters of the Galactic bulge (NGC 6541,NGC 6642,NGC 6681,NGC 6717,and NGC 6723) and also for three halo clusters.…”