On the Anomalous Balmer Line Strengths in Globular Clusters

TL;DR

This paper investigates the unexpectedly weak Balmer lines in globular clusters, ruling out abundance effects and suggesting stellar phenomena like M giant flaring as a likely cause.

Contribution

It identifies flaring in M giants as a plausible explanation for the anomalous Balmer line strengths in globular clusters.

Findings

Abundance mixture effects are unlikely causes.

Horizontal branch and blue straggler effects are insufficient.

Flaring in M giants is a favored explanation.

Abstract

Spectral feature index diagrams with integrated globular clusters and simple stellar population models often show that some clusters have weak H beta, so weak that even the oldest models cannot match the observed feature depths. In this work, we rule out the possibility that abundance mixture effects are responsible for the weak indices unless such changes operate to cool the entire isochrone. We discuss this result in the context of other explanations, including horizontal branch morphology, blue straggler populations, and nebular or stellar emission fill-in, finding a preference for flaring in M giants as an explanation for the H beta anomaly.