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.