On the roles of stellar rotation and binarity in NGC 2423's main-sequence turnoff region

Abstract: Research has shown that many young and intermediate-age clusters (younger than $\sim$2 Gyr) have extended main sequences and main-sequence turnoffs (eMSTOs), which cannot be adequately described by a single isochrone. The reason for the extended main sequences is now known, with the most probable cause being the fast rotation of stars. However, a significant fraction of slowly rotating stars form a younger stellar population than their fast-rotating counterparts, leading to speculation that they have undergone thorough rotational mixing processes internally. One speculation is that a considerable number of slowly rotating stars reside in close binary systems, where tidal forces from companion stars are the cause of their rotational deceleration. In this work, we report a relatively old open star cluster in the Milky Way, NGC 2423 ($\sim$1 Gyrs old), which exhibits an apparent eMSTO. As anticipated, many characteristics of NGC 2423 indicate that its eMSTO is driven by stellar rotations. Our calculations indicate that if slowly rotating stars commonly have a close companion star, they should exhibit significant differences in radial velocities observationally, and binary systems that can be tidally locked within the age of NGC 2423 should have a mass ratio close to 1. However, none of these predictions align with our observations. Interestingly, among the only two equal-mass binary systems in the observed region for which spectroscopic data could be obtained, we discovered that one of them is a tidally locked binary system. This further suggests the validity of our numerical simulation results.