WIYN Open Cluster Study. XVIII. The Tidal Circularization Cutoff Period of the Old Open Cluster NGC 188

Based on a sample of 22 main-sequence solar-type spectroscopic binaries, we find the tidal circularization cutoff in the open cluster NGC 188 (age 7 Gyr) to be well defined at a period of 15.0 days. When compared to shorter tidal circularization cutoff periods in the younger clusters M67 and the Hyades, this result establishes that tidal circularization processes are effective in solar-type main-sequence binaries for timescales longer than 1 Gyr. This result is in contrast to the theoretical prediction by Zahn & Bouchet that tidal circularization due to equilibrium tides is ineffective for late-type main-sequence stars, and that tidal circularization cutoff periods are instead set only by pre-main-sequence tidal circularization. The NGC 188 cutoff period is also longer than can be explained by present theories of tidal circularization due to dynamical tides.