Multiple stellar populations in three rich Large Magellanic Cloud star clusters

TL;DR

This study reveals that three large star clusters in the Large Magellanic Cloud contain multiple stellar populations with different ages, challenging traditional views of star cluster formation.

Contribution

It provides the first detailed evidence of multiple stellar populations in intermediate-age LMC clusters using deep colour-magnitude diagrams.

Findings

All three clusters show peculiar main-sequence turn-offs.

Two clusters have two distinct turn-off branches.

The age difference between populations is up to ~300 Myr.

Abstract

We present deep colour-magnitude diagrams for three rich intermediate-age star clusters in the Large Magellanic Cloud, constructed from archival ACS F435W and F814W imaging. All three clusters exhibit clear evidence for peculiar main-sequence turn-offs. NGC 1846 and 1806 each possess two distinct turn-off branches, while the turn-off for NGC 1783 shows a much larger spread in colour than can be explained by the photometric uncertainties. We demonstrate that although all three clusters contain significant populations of unresolved binary stars, these cannot be the underlying cause of the observed turn-off morphologies. The simplest explanation is that each cluster is composed of at least two different stellar populations with very similar metal abundances but ages separated by up to ~300 Myr. The origin of these unusual properties remains unidentified; however, the fact that at least three massive clusters containing multiple stellar populations are now known in the LMC suggests a potentially significant formation channel.