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In addition to the extended main-sequence turnoffs widely found in young andintermediate-age (~ 600 Myr-2 Gyr-old) star clusters, some younger clusterseven exhibit split main sequences (MSs). Different stellar rotation rates areproposed to account for the bifurcated MS pattern, with red and blue MSs (rMSand bMS) populated by fast and slowly rotating stars, respectively. Usingphotometry from Gaia Early Data Release 3, we report a Galactic open clusterwith a bifurcated MS, NGC 2422 ( ~ 90 Myr). We exclude the possibilities thatthe bifurcated MS pattern is caused by photometric noise or differentialreddening. We aim to examine if stellar rotation can account for the split MSs.We use spectra observed with the Canada-France-Hawaii Telescope and theSouthern African Large Telescope, and directly measured v sin i, the projectedrotational velocities, for stars populating the bMS and rMS. We find that theirv sin i values are weakly correlated with their loci in the color-magnitudediagram because of contamination caused by a large fraction of rMS stars withlow projected rotational velocities. Based on the spectral energy distributionfitting method, we suggest that these slowly rotating stars at the rMS may hidea binary companion, which breaks the expected v sin i-color correlation. Futuretime-domain studies focusing on whether these slowly rotating stars are radialvelocity variables are crucial to test the roles of stellar rotation andbinarity in generating the split MSs.

The role of binarity and stellar rotation in the split main sequence of NGC 2422