Multiple populations in globular clusters: Unified efforts from stellar evolution and chemical evolution models

TL;DR

This paper integrates stellar evolution and chemical evolution models to better understand multiple populations in globular clusters, linking HB morphology and chemical patterns to star formation history and subpopulation properties.

Contribution

It presents a unified modeling approach that combines synthetic HB and chemical evolution models to constrain the formation and chemical enrichment history of globular clusters.

Findings

Consistent results obtained for four GCs (M4, M5, M15, M80) using combined models.

He and Na abundances increase over generations, explaining observed diversity.

Models requiring dilution do not naturally explain the diversity observed in HB and chemical patterns.

Abstract

Recent stellar evolution models for globular clusters (GCs) in multiple population paradigm suggest that horizontal-branch (HB) morphology and mean period of type ab RR Lyrae variables are mostly determined by He & CNO abundances and relative ages for subpopulations. These parameters are also provided by chemical evolution models constructed to reproduce the Na-O anti-correlation. Therefore, a consistency check is possible between the synthetic HB and chemical evolution models. Furthermore, by combining them, a better constraint might be attained for star formation history and chemical abundances of subpopulations in GCs. We find, from such efforts made for four GCs, M4, M5, M15, and M80, that consistent results can be obtained from these two independent studies. In our unified model, He and Na abundances gradually increase over the generation, and therefore, the various extensions observed in both HB morphology and Na-O chemical pattern depend on the presence of later generation stars after the second generation. It is schematically shown that this observed diversity, however, would not be naturally explained by the models requiring dilution. Further spectroscopic observations are required, for metal-poor GCs in particular, to obtain a more detailed constraint from this approach.