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