A high fraction of Be stars in young massive clusters: evidence for a large population of near-critically rotating stars

TL;DR

This study finds a high fraction of Be stars in young massive clusters, supporting the idea that many stars in these clusters are near critical rotation, which explains observed features in their color-magnitude diagrams.

Contribution

It provides observational evidence for a large population of near-critically rotating stars in young massive clusters, confirming theoretical predictions about stellar rotation distributions.

Findings

30-60% of stars are Be stars in the clusters

Supports the hypothesis of many stars rotating near critical velocity

Findings align with models explaining main sequence splitting

Abstract

Recent photometric analysis of the colour-magnitude diagrams (CMDs) of young massive clusters (YMCs) have found evidence for splitting in the main sequence and extended main sequence turn-offs, both of which have been suggested to be caused by stellar rotation. Comparison of the observed main sequence splitting with models has led various authors to suggest a rather extreme stellar rotation distribution, with a minority ($10-30$\%) of stars with low rotational velocities and the remainder ($70-90$\%) of stars rotating near the critical rotation (i.e., near break-up). We test this hypothesis by searching for Be stars within two YMCs in the LMC (NGC 1850 and NGC 1856), which are thought to be critically rotating stars with decretion disks that are (partially) ionised by their host stars. In both clusters we detect large populations of Be stars at the main sequence turn-off ($\sim30-60$\% of stars), which supports previous suggestions of large populations of rapidly rotating stars within massive clusters.