The effects of stellar rotation along the main sequence of the 100 Myr old massive cluster NGC 1850

TL;DR

This study combines spectroscopy and photometry to analyze stellar rotation effects in the 100 Myr-old cluster NGC 1850, revealing correlations between rotation speed, color, and the presence of Be stars, informing stellar evolution models.

Contribution

It provides the first detailed spectroscopic measurement of stellar rotation in NGC 1850, linking rotation rates to split main sequence features and identifying a significant population of Be stars with decretion disks.

Findings

Fast rotators are redder and have higher Vsini (~200 km/s)

Approximately 23% of Be stars show shell features indicating decretion disks

Rotation rates in cluster stars are comparable to field stars of similar mass

Abstract

Young star clusters enable us to study the effects of stellar rotation on an ensemble of stars of the same age and across a wide range in stellar mass and are therefore ideal targets for understanding the consequences of rotation on stellar evolution. We combine MUSE spectroscopy with HST photometry to measure the projected rotational velocities (Vsini) of 2,184 stars along the split main sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive (10^5 M_sun) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed magnitude, we observe a clear correlation between Vsini and colour, in the sense that fast rotators appear redder. The average Vsini values for stars on the blue and red branches of the split main sequence are ~100 km/s and ~200 km/s, respectively. The values correspond to about 25-30% and 50-60% of the critical rotation velocity and imply that rotation rates comparable to those observed in field stars of similar masses can explain the split main sequence. Our spectroscopic sample contains a rich population of ~200 fast rotating Be stars. The presence of shell features suggests that 23% of them are observed through their decretion disks, corresponding to a disk opening angle of 15 degrees. These shell stars can significantly alter the shape of the MSTO, hence care should be taken when interpreting this photometric feature. Overall, our findings impact our understanding of the evolution of young massive clusters and provide new observational constraints for testing stellar evolutionary models.