Three Parameters for the Horizontal-Branch Morphology in Globular Clusters

TL;DR

This study analyzes the factors influencing the horizontal-branch morphology in globular clusters, highlighting the roles of metallicity, age, mass, and helium variation in shaping the observed features.

Contribution

It identifies three key parameters—metallicity, age, and helium variation—that govern the HB morphology in globular clusters, advancing understanding beyond metallicity alone.

Findings

HB color extension correlates with cluster mass

Color distance between HB and RGB depends on metallicity and age

Age and metallicity are primary global parameters for HB morphology

Abstract

The horizontal branch (HB) morphology of globular clusters (GCs) is mainly described by metallicity. The fact that some clusters with almost the same metallicity exhibit different HB demonstrates that other parameters are at work. We present results from the analysis of the CMD of 72 GCs obtained with the Advanced Camera for Survey (ACS) of the Hubble Space Telescope (HST). We find a significant correlation between the HB color extension and the mass of the hosting cluster, while the color distance between the HB and the red-giant branch (RGB) depends on metallicity and age. We suggest that age and metallicity are the main global parameters of the HB morphology in GCs, while the HB extension is mainly due to internal helium variation, associated to multiple stellar populations.