Investigating Extended Main-Sequence Turnoffs in Galactic Open Clusters

TL;DR

This study surveys 53 galactic open clusters to understand the roles of stellar rotation and extinction in the extended main sequence turnoff phenomena, revealing diverse cluster behaviors and quantifying stellar rotation fractions.

Contribution

It provides a comprehensive analysis of the factors influencing eMSTO in open clusters, including the effects of stellar rotation and differential extinction, with new classification and quantification methods.

Findings

14 clusters show split main sequences with distinct rotation populations.

Differential extinction affects MSTO width and age estimates.

Median slow rotator fraction among MSTO stars is approximately 0.41.

Abstract

The extended main sequence (eMS) and extended main sequence turnoff (eMSTO) phenomena have been observed in some young and intermediate-age star clusters in the Milky Way and in the Magellanic Clouds. In this study, we conduct a survey of 53 galactic open clusters (OCs) to investigate the roles of stellar rotation, differential extinction, and cluster properties in the emergence of eMS and eMSTO. The projected rotational velocities are taken from the Gaia ESO spectroscopic survey and the Gaia DR3 line-broadening velocities. Stellar members of each OC are identified using the ML-MOC algorithm with Gaia DR3 astrometry. We divide clusters into four classes based on the color-rotation distribution, extinction, and MSTO morphology and report 14 clusters ($A_{\rm v} \lesssim 0.15$ mag, Class I) that exhibit split MS with fast and slow rotators populating the redder and bluer parts of MSTO. For the remaining clusters, differential extinction hampers the color-rotation distinction and also inflates MSTO width and therefore introduces a systematic offset in the MSTO-age relation. We also quantify the fraction of slow rotators among MSTO stars, finding a median value of $f_{\rm slow\, rot}^{v \sin i<100} \approx 0.41$ and the fraction reaching the spin-down limit, $f_{\rm slow\, rot}^{v \sin i<30}$, is $ \approx 0.08$. We find no statistically significant correlation between $f_{\rm slow\, rot}$ and either the binary fraction or cluster age.