Red giant branch in near-infrared colour-magnitude diagrams - I. Calibration of photometric indices

Abstract

We present new high-quality near-infrared photometry of 10 Galactic globular clusters spanning a wide metallicity range (-2.12 ≤ [Fe/H] ≤ - 0.49): five clusters belong to the halo (NGC 288, 362, 6752, M15 and M30) and five (NGC 6342, 6380, 6440, 6441 and 6624) to the bulge. By combining J, H and K observations with optical data, we constructed colour-magnitude diagrams in various planes: (K, J-K), (K, V-K), (H, J-H) and (H, V-H). A set of photometric indices (colours, magnitudes and slopes) describing the location and the morphology of the red giant branch (RGB) have been measured. We have combined this new data set with those collected by our group over the last 5 years, and here we present an updated calibration of the various RGB indices in the Two-Micron All-Sky Survey photometric system, in terms of the cluster metallicity.