2026ApJ..1000..131W

Tidal interactions in close binaries play a key role in the long-term rotational and orbital evolution. The distributions of circularization across open clusters (OCs) place strong observational constraints on tidal dissipation in binaries. However, direct observational constraints on synchronization among binaries in OCs remain limited. For the 125 Myr OC Pleiades, this work combines cluster membership from Gaia Data Release 3, rotation periods from the K2 mission, and orbital solutions of the binary population from a long-term spectroscopic survey to investigate the degree of tidal synchronization in each binary by comparing the pseudosynchronization period to the rotation period of the primary stars. Among 42 binaries with reliable orbital periods P_orb and rotation periods, we identify seven tidally synchronized systems with P_orb ≲ 8.6 days, including one early-type system and six late-type systems. For binaries with longer P_orb, primaries generally are supersynchronized, and most systems are eccentric. We find a synchronization transition near P_orb ≈ 8.6-14 days, comparable to the known circularization period (P_orb ≈ 7.2 days) in the Pleiades, which suggests similar critical period scales for synchronization and circularization in this coeval population. Synchronization depends much more strongly on mass ratio than on primary mass. Most synchronized systems in Pleiades have high mass ratios and are likely to evolve into double white dwarf systems. Tides likely impose strong rotational braking on close early-type binaries, while their influence on late-type close binaries is weaker, and their spins largely follow the single-star sequence.