WIYN Open Cluster Study: Tidal Interactions in Solar type Binaries

We present an ongoing study on tidal interactions in late-type close binary stars. New results on tidal circularization are combined with existing data to test and constrain theoretical predictions of tidal circularization in the premain-sequence (PMS) phase and throughout the main-sequence phase of stellar evolution. Current data suggest that tidal circularization during the PMS phase sets the tidal cutoff period for binary populations younger than ∼ 1 Gyr. Binary populations older than ∼ 1 Gyr show increasing tidal cutoff periods with age, consistent with active main-sequence tidal circularization. 1. The effects of tidal interactions Recent studies of angular momentum gain and loss in young solar-type stars have primarily been focused on single stars and the interaction with circumstellar disks (e.g. Barnes et al. 2001, Terndrup et al. 2000, and Krishnamurthi et al. 1997). Motivated by the high frequency of late-type binary stars, we will study the affect of close binary companions on stellar angular momentum evolution through tidal interactions. Tidal deformation of stars in binaries results in a torque component in their gravitational attraction. This torque is responsible for the spin-orbit coupling by which angular momentum is exchanged between the star and the orbit (Zahn 1966,1977, Hut 1981 and references therein). Tidal interactions will continually change the rotation of the binary components and the orbital parameters of the binary system until an equilibrium state is reached. An equilibrium state is characterized by co-planarity of the equatorial planes of the two stars with the orbital plane of the system, circularity of the stellar orbits, and synchronization of the stellar rotation with the orbital revolution of the system. Figure 1 (Zahn & Bouchet 1989) shows an example of a theoretical model of tidal evolution from the birth-line through the main-sequence phase for a binary system of two 1 M components. In this model, the friction that the tidal bulge experiences as the star rotates is caused by the interaction between convective turbulence and the tidal flow. The turbulent viscosity is reduced when the orbital period becomes shorter than the convective turnover time. The torque (τ) on the tidal bulge is a sensitive function of the ratio of the stellar radius (R) to the distance r between the stars (τ ∝ ( R r )). The timescales for tidal evolution are therefore extremely sensitive to the stellar radius and the