Tidal-locking-induced stellar rotation dichotomy in the open cluster NGC 2287?

TL;DR

This study links the double main sequence observed in the young open cluster NGC 2287 to a dichotomy in stellar rotation rates, suggesting tidal locking in binary systems influences stellar rotation and the cluster's color-magnitude diagram features.

Contribution

It provides the first direct evidence connecting double main sequences in a young cluster to a bimodal distribution of stellar rotation rates, highlighting the role of tidal locking in stellar rotation evolution.

Findings

Double MS in NGC 2287 correlates with two distinct rotation rate groups.

Slow rotators may be tidally locked in binary systems with low mass ratios.

Stellar rotation influences the appearance of the main sequence in young clusters.

Abstract

Stars spend most of their lifetimes on the `main sequence' (MS) in the Hertzsprung--Russell diagram. The obvious double MSs seen in the equivalent color--magnitude diagrams characteristic of Milky Way open clusters pose a fundamental challenge to our traditional understanding of star clusters as `single stellar populations.' The clear MS bifurcation of early-type stars with masses greater than $\sim1.6 M_\odot$ is thought to result from a range in the stellar rotation rates. However, direct evidence connecting double MSs to stellar rotation properties has yet to emerge. Here, we show through analysis of the projected stellar rotational velocities ($v\sin i$, where $i$ represents the star's inclination angle) that the well-separated double MS in the young, $\sim200Myr$-old Milky Way open cluster NGC 2287 is tightly correlated with a dichotomous distribution of stellar rotation rates. We discuss whether our observations may reflect the effects of tidal locking affecting a fraction of the cluster's member stars in stellar binary systems. We show that the slow rotators could potentially be initially rapidly rotating stars that have been slowed down by tidal locking by a low mass-ratio companion in a cluster containing a large fraction of short-period, low-mass-ratio binaries. This demonstrates that stellar rotation drives the split MSs in young, $\lessapprox 300$Myr-old star clusters. However, special conditions, e.g., as regards the mass-ratio distribution, might be required for this scenario to hold.