The population of M-type supergiants in the starburst cluster Stephenson 2

TL;DR

This study analyzes the red supergiant population in the starburst cluster Stephenson 2, revealing spectral type distinctions and evidence linking late-M types to heavy mass loss, advancing understanding of stellar evolution in massive clusters.

Contribution

It provides detailed spectral analysis of over 30 RSGs in Stephenson 2, highlighting the association between late-M spectral types and heavy mass loss, a novel insight into stellar evolution.

Findings

Clear spectral type separation among RSGs in Stephenson 2

Late-M spectral types show strong evidence of heavy mass loss

Mass loss and maser emission linked to late-M spectral types

Abstract

The open cluster Stephenson 2 contains the largest collection of red supergiants known in the Galaxy, and at present is the second most massive young cluster known in the Milky Way. We have obtained multi-epoch, intermediate-resolution spectra around the CaII triplet for more than 30 red supergiants in Stephenson~2 and its surroundings. We find a clear separation between a majority of RSGs having spectral types M0-M2 and the brightest members in the NIR, which have very late spectral types and show strong evidence for heavy mass loss. The distribution of spectral types is similar to that of RSGs in other clusters, such as NGC 7419, or associations, like Per OB1. The cluster data strongly support the idea that heavy mass loss and maser emission is preferentially associated with late-M spectral types, suggesting that they represent an evolutionary phase.