Rotation Period Evolution in Low-Mass Binary Stars: The Impact of Tidal Torques and Magnetic Braking
David P. Fleming, Rory Barnes, James R. A. Davenport, and Rodrigo Luger
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 d across a wide range tidal dissipation parameters using two common equilibrium tidal models. We find that many binaries with Porb ~20 d tidally-lock, and most with Porb < 4 d 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 Gyrs, even in short Porb binaries, qualitatively reproducing the subsynchronous eclipsing binaries (EBs) discovered in the Kepler field by Lurie et al. (2017). Both equilibrium tidal models predict that binaries can tidally-interact out to Porb ~80 d, while the CPL equilibrium tidal model predicts that binaries can tidally-lock out to Porb ~100 d. 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 be a valid proxy for age in all cases, i.e. gyrochronology methods can fail 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 describes tidal interactions in low-mass binary stars.