Interpreting the complex CMDs of the Magellanic Clouds clusters

TL;DR

This paper reviews the complex features observed in the color-magnitude diagrams of Magellanic Clouds star clusters, linking phenomena like rotation and abundance variations to different age groups and cluster properties.

Contribution

It provides a comprehensive overview of current observations and interpretations of CMD complexities in Magellanic Clouds clusters, highlighting links between different phenomena across ages.

Findings

Young clusters show extended main sequence turnoffs and split main sequences.

Older clusters exhibit red giant branch splitting linked to abundance variations.

Stellar rotation and light-element abundance variations explain CMD features.

Abstract

The Magellanic Clouds host a large population of massive (> 10^4 Msun) star clusters with ages ranging from a few Myr to 12 Gyr. In nearly all cases, close inspection of their CMDs reveals features that deviate from expectations of a classic isochrone. Young (< 2 Gyr) clusters show extended main sequence turnoffs and in some cases split/dual main sequences. Clusters older than ~ 2 Gyr show splitting in the red giant branches when viewed in UV filters that are sensitive to abundance variations (in particular nitrogen). A distribution of stellar rotation rates appears to be the cause of the complex features observed in the young and intermediate age clusters, while above ~ 2 Gyr the features seem to be the same light-element abundance variations as observed in the ancient Galactic globular clusters, a.k.a. "multiple populations". Here, we provide an overview of current observations and their interpretations and summarise possible links between all the classes of complexities, regardless of age.