Multimodal Horizontal Branches: Empirical Evidence and Possible Evolutionary Scenarios

TL;DR

This paper reviews empirical evidence and theoretical scenarios regarding the complex features of horizontal branch stars in globular clusters, highlighting the evolution of observational data and the implications for stellar evolution models.

Contribution

It synthesizes recent observational and theoretical developments on HB star distributions, emphasizing the increasing evidence for multimodal HBs and their early cluster origins.

Findings

Gaps along the HB are less prominent with better data.

Multimodal HBs have become more evident.

Multiple components are traced to early cluster processes.

Abstract

We review the available empirical evidence for the presence of "gaps" and multimodal distributions among horizontal branch (HB) stars, along with some of the theoretical scenarios that have been proposed to explain these features. While gaps along the HB have become increasingly less prominent and frequent as more and better color-magnitude diagram data have been obtained for Galactic globular clusters, the evidence for multimodal HBs has instead become stronger. In addition, different HB modes have recently started to be traced down to multiple components that have been detected among subgiant branch and main sequence stars, thus suggesting that their origin lies in the complex physical processes that took place at the earliest stages in the history of massive stellar clusters.