Clusters rich in red supergiants

TL;DR

Recent discoveries of massive clusters rich in red supergiants in the Milky Way provide valuable insights into their evolution, properties, and the characteristics of late-M type supergiants, despite observational challenges.

Contribution

This paper reviews the properties and significance of newly discovered red supergiant clusters, highlighting their role in understanding stellar evolution and the nature of late-M supergiants.

Findings

Clusters contain large, homogeneous populations of red supergiants.

Spectral-type distribution peaks at M1 among Milky Way red supergiants.

Evidence supports late-M supergiants as a distinct class with heavy mass loss.

Abstract

In the past few years, several clusters containing large numbers of red supergiants have been discovered. These clusters are amongst the most massive young clusters known in the Milky Way, with stellar masses reaching a few $10^4\:$M$_\odot$. They have provided us, for the first time, with large homogeneous samples of red supergiants of a given age. These large populations make them, despite heavy extinction along their sightlines, powerful laboratories to understand the evolutionary status of red supergiants. While some of the clusters, such as the eponymous RSGC1, are so obscured that their members are only observable in the near-IR, some of them are easily accessible, allowing for an excellent characterisation of cluster and stellar properties. The information gleaned so far from these clusters gives strong support to the idea that late-M type supergiants represent a separate class, characterised by very heavy mass loss. It also shows that the spectral-type distribution of red supergiants in the Milky Way is very strongly peaked towards M1, while suggesting a correlation between spectral type and evolutionary stage.