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Stars spend most of their lifetimes on the `main sequence' (MS) in theHertzsprung--Russell diagram. The obvious double MSs seen in the equivalentcolor--magnitude diagrams characteristic of Milky Way open clusters pose afundamental challenge to our traditional understanding of star clusters as`single stellar populations.' The clear MS bifurcation of early-type stars withmasses greater than $\sim1.6 M_\odot$ is thought to result from a range in thestellar rotation rates. However, direct evidence connecting double MSs tostellar rotation properties has yet to emerge. Here, we show through analysisof the projected stellar rotational velocities ($v\sin i$, where $i$ representsthe 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 adichotomous distribution of stellar rotation rates. We discuss whether ourobservations may reflect the effects of tidal locking affecting a fraction ofthe cluster's member stars in stellar binary systems. We show that the slowrotators could potentially be initially rapidly rotating stars that have beenslowed down by tidal locking by a low mass-ratio companion in a clustercontaining a large fraction of short-period, low-mass-ratio binaries. Thisdemonstrates that stellar rotation drives the split MSs in young, $\lessapprox300$Myr-old star clusters. However, special conditions, e.g., as regards themass-ratio distribution, might be required for this scenario to hold.

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