On the Delta V_HB_bump parameter in Globular Clusters

TL;DR

This study provides new empirical measurements of the Delta V_HB_bump parameter in 15 globular clusters, revealing discrepancies with theoretical models across a broad metallicity range, especially in metal-poor clusters.

Contribution

It offers the first homogeneous empirical estimates of Delta V_HB_bump for 15 clusters and compares them with models, highlighting persistent discrepancies.

Findings

Observed Delta V_HB_bump values are larger than predicted in metal-poor clusters.

Approximately 40% of metal-poor clusters show significant (~0.40 mag) deviations.

Evolutionary models with alpha-, CNO-, or helium-enhancement do not resolve the discrepancies.

Abstract

We present new empirical estimates of the Delta V_HB_bump parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16<=[M/H]<=-0.58 dex). Adopting the homogeneous metallicity scale provided either by Kraft & Ivans (2004) or by Carretta et al. (2009), we found that the observed Delta V_HB_bump parameters are larger than predicted. In the metal-poor regime ([M/H]<=-1.7, -1.6 dex) 40% of GCs show discrepancies of 2sigma (~0.40 mag) or more. Evolutionary models that account either for alpha- and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations. The outcome is the same if we use the new Solar heavy-element mixture. The comparison between alpha- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed Delta V_HB_bump parameters, in the metal-rich regime ([M/H]=>0), might be systematically smaller than predicted.