The Red Giant Branch in Near - Infrared Colour - Magnitude Diagrams . I : The calibration of photometric indices ⋆

We present new high-quality near-infrared photometry of 10 Galactic Globular Clusters spanning a wide metallicity range (−2.126[Fe/H]6 − 0.49): five clusters belong to the Halo (namely, NGC 288, NGC 362, NGC 6752, M 15 and M 30) and five (namely, NGC 6342, NGC 6380, NGC 6440, NGC 6441 and NGC 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 already published by Ferraro et al. (2000) and Valenti et al. (2004), and here we present an updated calibration of the various RGB indices in the 2MASS photometric system, in terms of the cluster metallicity.