A new infrared array photometric survey of galactic globular clusters: A detailed study of the red giant branch sequence as a step toward the global testing of stellar models

We present high-quality near-infrared color-magnitude diagrams of 10 Galactic globular clusters (GCs) spanning a wide metallicity range (-2.15 < [Fe/H] < -0.2). This homogeneous database has been used to perform a detailed analysis of the red giant branch (RGB), adopting a variety of observables to describe its physical and chemical properties. First, a set of metallicity indicators have been measured, namely, (1) the RGB (J - K) and (V - K) colors at different magnitude levels, (2) the RGB K magnitude at different colors, and (3) the RGB slope. For these parameters we present new calibrations in terms of both spectroscopic iron abundance and global metallicity, including the alpha-element enhancement. These relations can be used to derive a photometric estimate of the GC metal content from the RGB morphology and location. Second, the location in luminosity of the main RGB features (namely, the bump and the tip) and their dependence on metallicity have been studied, yielding quantitative observational relationships. Finally, adopting new transformations between the observational and theoretical quantities, the mean ridge lines for the clusters of our sample have been reported in the [M-Bol, log (T-e)] plane. This allows us to study the RGB location in terms of effective temperature, bolometric luminosity of the main RGB features, and their calibrations with varying metallicity. Direct comparisons between updated theoretical models and observations show an excellent overall agreement.