2000AJ....119.2274I

Infrared JK_s-band photometry of the Galactic globular clusters M15 and, for the first time, M56 is presented. We estimate the reddening [E(B-V)=0.18±0.08 mag] and distance modulus [(m-M)_V=15.43±0.30 mag] toward the poorly studied globular cluster M56. We combine our data with observations of other clusters from the literature (12 in total) to extend the [Fe/H] versus red giant branch (RGB) slope relation toward metal-poor clusters. Our best fit yields [Fe/H]=-3.40(±0.22)-27.74(±2.35)(RGB slope), with rms=0.20. The broader metallicity baseline greatly reduced the uncertainties compared with other existing calibrations. We confirm a previously obtained calibration of the relation between the RGB color (J-K_s)₀(RGB) at M_Ks=-5.5 versus [Fe/H]: [Fe/H]=-6.90(±0.99)+6.63(±1.05)(J-K_s)₀(RGB) with rms=0.33. Finally, using the new RGB slope calibration we estimated the abundance of the super-metal-rich cluster Liller 1: [Fe/H]=0.34±0.22.