paper

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

arXiv:1903.05686 · doi:10.3847/1538-4357/ab2ed2

Abstract

We examine how tides, stellar evolution, and magnetic braking shape the rotation period (P $_{r o t}$ ) evolution of low-mass stellar binaries up to orbital periods (P $_{or b}$ ) of 100 d across a wide range tidal dissipation parameters using two common equilibrium tidal models. We find that many binaries with P $_{or b} < 20$ d tidally lock, and most with $P_{or b} < 4$ d tidally lock into synchronous rotation on circularized orbits. At short P $_{or b}$ , 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 P $_{or b}$ 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 P $_{or b} \approx 80$ d, while the Constant Phase Lag tidal model predicts that binaries can tidally lock out to P $_{or b} \approx 100$ d. Tidal torques often force the P $_{r o t}$ evolution of stellar binaries to depart from the long-term magnetic braking-driven spin down experienced by single stars, revealing that P $_{r o t}$ is not be 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 P $_{r o t}$ can be used to discriminate between which equilibrium tidal models best describes tidal interactions in low-mass binary stars.

Accepted, ApJ