Star Clusters in M31. V. Evidence for Self-Enrichment in Old M31 Clusters from Integrated Spectroscopy

TL;DR

This study analyzes the chemical abundances in 72 old globular clusters in M31, revealing a correlation between cluster mass and nitrogen enrichment, which informs models of their formation and chemical evolution.

Contribution

It provides the first evidence of a mass-abundance correlation in M31 GCs, highlighting the role of self-enrichment in their formation history.

Findings

Correlation between cluster mass and nitrogen abundance.

Evidence supporting self-enrichment processes in GCs.

Constraints on globular cluster formation models.

Abstract

In the past decade, the notion that globular clusters (GCs) are composed of coeval stars with homogeneous initial chemical compositions has been challenged by growing evidence that they host an intricate stellar population mix, likely indicative of a complex history of star formation and chemical enrichment. Several models have been proposed to explain the existence of multiple stellar populations in GCs, but no single model provides a fully satisfactory match to existing data. Correlations between chemistry and global parameters such as cluster mass or luminosity are fundamental clues to the physics of GC formation. In this Letter, we present an analysis of the mean abundances of Fe, Mg, C, N, and Ca for 72 old GCs from the Andromeda galaxy. We show for the first time that there is a correlation between the masses of GCs and the mean stellar abundances of nitrogen, spanning almost two decades in mass. This result sheds new light on the formation of GCs, providing important constraints on their internal chemical evolution and mass loss history.