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