Wide Field Near-Infrared Photometry of 12 Galactic Globular Clusters: Observations Versus Models on the Red Giant Branch

TL;DR

This study provides near-infrared photometry of 12 Galactic globular clusters, compares observations with stellar models, recalibrates photometric relations, and examines how various factors influence RGB and HB features.

Contribution

It offers the first comprehensive comparison of near-IR cluster data with recent stellar models and recalibrates empirical relations for RGB indices versus metallicity.

Findings

Dartmouth and Victoria-Regina models best match observed RGB morphology.

Recalibrated relations between IR indices and metallicity are consistent with previous studies.

Near-IR HB magnitude shows a significant dependence on cluster metallicity.

Abstract

We present wide field near-infrared photometry of 12 Galactic globular clusters, typically extending from the tip of the cluster red giant branch (RGB) to the main sequence turnoff. Using recent homogenous values of cluster distance, reddening and metallicity, the resulting photometry is directly compared to the predictions of several recent libraries of stellar evolutionary models. Of the sets of models investigated, Dartmouth and Victoria-Regina models best reproduce the observed RGB morphology, albeit with offsets in J-Ks color which vary in their significance in light of all sources of observational uncertainty. Therefore, we also present newly recalibrated relations between near-IR photometric indices describing the upper RGB versus cluster iron abundance as well as global metallicity. The influence of enhancements in alpha elements and helium are analyzed, finding that the former affect the morphology of the upper RGB in accord with model predictions. Meanwhile, the empirical relations we derive are in good agreement with previous results, and minor discrepancies can likely be attributed to differences in the assumed cluster distances and reddenings. In addition, we present measurements of the horizontal branch (HB) and RGB bump magnitudes, finding a non-negligible dependence of the near-IR HB magnitude on cluster metallicity. Lastly, we discuss the influence of assumed cluster distances, reddenings and metallicities on our results, finding that our empirical relations are generally insensitive to these factors to within their uncertainties.