Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations

TL;DR

This study provides spectroscopic evidence of multiple stellar populations in the intermediate age cluster Kron 3 in the Small Magellanic Cloud, challenging previous notions that such features only occur in older globular clusters.

Contribution

It is the first to identify multiple populations in a 6.5 Gyr old cluster, extending the age range of clusters known to host multiple populations.

Findings

Detection of nitrogen-enriched sub-population in Kron 3

Multiple populations present in a 6.5 Gyr old cluster

Implication that multiple populations can form at later cosmic times

Abstract

We present the results of a spectroscopic study of the intermediate age (approximately 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at 3839 and 4300 Angstroms respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of 5 stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3.