Evidence for multiple populations in the intermediate age cluster Lindsay 1 in the SMC

TL;DR

This study provides evidence of multiple stellar populations in the intermediate-age cluster Lindsay 1 in the SMC, based on nitrogen enrichment in its stars, indicating such populations can form at least until redshift ~1.

Contribution

It is the first to identify multiple populations in a cluster as young as Lindsay 1, showing that these phenomena occur at earlier cosmic times than previously known.

Findings

Detection of nitrogen-enriched sub-population in Lindsay 1

Exclusion of evolutionary mixing as cause of nitrogen variation

Implication that multiple populations form at redshift ~1

Abstract

Lindsay 1 is an intermediate age (approx 8 Gyr) massive cluster in the Small Magellanic Cloud (SMC). Using VLT FORS2 spectra of 16 probable cluster members on the lower RGB of the cluster, we measure CN and CH band strengths (at 3883 and 4300 Angstroms respectively), along with carbon and nitrogen abundances and find that a sub-population of stars has significant nitrogen enrichment. A lack of spread in carbon abundances excludes evolutionary mixing as the source of this enrichment, so we conclude that this is evidence of multiple populations. Therefore, L1 is the youngest cluster to show such variations, implying that the process triggering the onset of multiple populations must operate until at least redshift ~1.