Synchronization Timescales for Three Solar-Type Stars That Have Jupiter-Mass Companions in Short-Period Orbits

We examine the physical properties of the three currently known star-plus-planet systems that have orbital periods of ∼3–5 days (51 Peg, υ And, and τ Boo). The observed properties are, in general, more consistent with the Zahn viscous dissipation mechanism than with the Tassoul hydrodynamic spin-down mechanism (at least in the latter's present formulation). However, a comparison of the data for τ Boo with the Zahn theory suggests that the predicted synchronization timescale t Z might be regarded as an upper limit to the actual time needed to synchronize the outer convective layers, and that the latter may actually be shorter than t Z by a factor of ≥3.