The split main sequences found in the colour-magnitude diagrams of star
clusters younger than ~600 Myr are suggested to be caused by the dichotomy of
stellar rotation rates of upper main-sequence stars. Tidal interactions have
been suggested as a possible explanation of the dichotomy of the stellar
rotation rates. This hypothesis proposes that the slow rotation rates of stars
along the split main sequences are caused by tidal interactions in binaries. To
test this scenario, we measured the variations in the radial velocities of
slowly rotating stars along the split main sequence of the young Galactic
cluster NGC 2422 (~90 Myr) using spectra obtained at multiple epochs with the
Canada-France-Hawai'i Telescope. Our results show that most slowly rotating
stars are not radial-velocity variables. Using the theory of dynamical tides,
we find that the binary separations necessary to fully or partially synchronise
our spectroscopic targets, on time-scales shorter than the cluster age, predict
much larger radial velocity variations across multiple-epoch observations, or a
much larger radial velocity dispersion at a single epoch, than the observed
values. This indicates that tidal interactions are not the dominant mechanism
to form slowly rotating stars along the split main sequences. As the
observations of the rotation velocity distribution among B- and A-type stars in
binaries of larger separations hint at a much stronger effect of braking with
age, we discuss the consequences of relaxing the constraints of the dynamical
tides theory.Comment: 14 pages, 10 figures, 2 tables, accepted for publication in MNRA