  In addition to the extended main-sequence turnoffs widely found in young and
intermediate-age (~ 600 Myr-2 Gyr-old) star clusters, some younger clusters
even exhibit split main sequences (MSs). Different stellar rotation rates are
proposed to account for the bifurcated MS pattern, with red and blue MSs (rMS
and bMS) populated by fast and slowly rotating stars, respectively. Using
photometry from Gaia Early Data Release 3, we report a Galactic open cluster
with a bifurcated MS, NGC 2422 ( ~ 90 Myr). We exclude the possibilities that
the bifurcated MS pattern is caused by photometric noise or differential
reddening. We aim to examine if stellar rotation can account for the split MSs.
We use spectra observed with the Canada-France-Hawaii Telescope and the
Southern African Large Telescope, and directly measured v sin i, the projected
rotational velocities, for stars populating the bMS and rMS. We find that their
v sin i values are weakly correlated with their loci in the color-magnitude
diagram because of contamination caused by a large fraction of rMS stars with
low projected rotational velocities. Based on the spectral energy distribution
fitting method, we suggest that these slowly rotating stars at the rMS may hide
a binary companion, which breaks the expected v sin i-color correlation. Future
time-domain studies focusing on whether these slowly rotating stars are radial
velocity variables are crucial to test the roles of stellar rotation and
binarity in generating the split MSs.
