An Observational Study of Tidal Synchronization in Solar‐Type Binary Stars in the Open Clusters M35 and M34

Abstract

We present rotation periods for the solar-type primary stars in 13 close (a~0) with the orbital motion. Of the six closest binaries two with circular orbits are not synchronized, one being subsynchronous and one being supersynchronous, and the primary stars in two binaries with eccentric orbits are rotating more slowly than pseudosynchronism. The remaining two binaries have reached the equilibrium state of both a circularized orbit and synchronized rotation. As a set, the six binaries present a challenging case study for tidal evolution theory, which in particular does not predict subsynchronous rotation in such close systems.