Extending the Red Giant Branch versus Metallicity Calibration toward Metal-poor Systems: Near-Infrared Photometry of the Galactic Globular Clusters M56 and M15

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. Based on data taken at the Steward Observatory 2.3 m Bok Telescope equipped with the 256x256 near-IR camera.