Horizontal Branch Morphology and Mass Loss in Globular Clusters

TL;DR

This study demonstrates that alpha-enhancement and a simple mass loss-metallicity relation can explain the diverse horizontal branch morphologies in globular clusters, enhancing understanding of stellar evolution and population synthesis.

Contribution

It introduces synthetic HB models showing that metallicity and alpha-enhancement alone can account for HB morphology variations in old globular clusters.

Findings

Metal-poor GCs ([Fe/H] < -2) have redder HBs than slightly more metal-rich GCs.

A simple mass loss-metallicity relation explains HB morphology without invoking younger ages.

Results aid in stellar population synthesis and understanding of stellar evolution.

Abstract

The connection between mass loss on the red giant branch (RGB) and horizontal branch (HB) morphology in globular clusters (GCs) has long been acknowledged but the mechanisms governing mass loss remains poorly understood from a theoretical perspective. The present study uses synthetic HB models to demonstrate for the first time that alpha-enhancement and a simple relation between mass loss and metallicity can explain the entire range of HB morphology (characterized by the HB type index) observed in old, coeval GCs. The mass loss-metallicity relation accounts naturally for the fact that the most metal poor GCs ([Fe/H] < -2) have redder HBs than is typical of GCs with -2 < [Fe/H] < -1.5 without invoking younger ages. These results may prove useful in studying the contribution of HB stars to integrated light via stellar population synthesis.