Rotation Period Evolution in Low-mass Binary Stars: The Impact of Tidal Torques and Magnetic Braking

We examine how tides, stellar evolution, and magnetic braking shape the rotation period (Prot) evolution of low-mass stellar binaries up to orbital periods (Porb) of 100 days across a wide range of tidal dissipation parameters using two common equilibrium tidal models. We find that many binaries with Porb 20 days tidally lock, and most with Porb 4 days tidally lock into synchronous rotation on circularized orbits. At short Porb, tidal torques produce a population of fast rotators that single-star-only models of magnetic braking fail to produce. In many cases, we show that the competition between magnetic braking and tides produces a population of subsynchronous rotators that persists for 1 Gyr, even in short-Porb binaries, qualitatively reproducing the subsynchronous eclipsing binaries discovered in the Kepler field by Lurie et al. Both equilibrium tidal models predict that binaries can tidally interact out to Porb 80 days, while the constant phase lag tidal model predicts that binaries can tidally lock out to Porb 100 days. Tidal torques often force the Prot evolution of stellar binaries to depart from the long-term magnetic-braking-driven spin-down experienced by single stars, revealing that Prot is not a valid proxy for age in all cases, i.e., gyrochronology can underpredict ages by up to 300% unless one accounts for binarity. We suggest that accurate determinations of orbital eccentricties and Prot can be used to discriminate between which equilibrium tidal models best describe tidal interactions in low-mass binary stars.