Tidal Synchronization and Differential Rotation of Kepler Eclipsing Binaries

Abstract

Few observational constraints exist for the tidal synchronization rate of late-type stars, despite its fundamental role in binary evolution. We visually inspected the light curves of 2278 eclipsing binaries (EBs) from the Kepler Eclipsing Binary Catalog to identify those with starspot modulations, as well as other types of out-of-eclipse variability. We report rotation periods for 816 EBs with starspot modulations, and find that 79% of EBs with orbital periods of less than 10 days are synchronized. However, a population of short-period EBs exists, with rotation periods typically 13% slower than synchronous, which we attribute to the differential rotation of highlatitude starspots. At 10 days, there is a transition from predominantly circular, synchronized EBs to predominantly eccentric, pseudosynchronized EBs. This transition period is in good agreement with the predicted and observed circularization period for Milky Way field binaries. At orbital periods greater than about 30 days, the amount of tidal synchronization decreases. We also report 12 previously unidentified candidate δ Scuti and γ Doradus pulsators, as well as a candidate RS CVn system with an evolved primary that exhibits starspot occultations. For short-period contact binaries, we observe a period-color relation and compare it to previous studies. As a whole, these results represent the largest homogeneous study of tidal synchronization of late-type stars.

Turbulent Viscosity Acting on the Equilibrium Tidal Flow in Convective Stars

“…Our results are directly relevant for interpreting observational evidence for synchronization and circularization of solar-type and low-mass close binaries (e.g. Meibom & Mathieu 2005;Meibom et al 2006;Van Eylen et al 2016;Lurie et al 2017;Triaud et al 2017). We can estimate the convective turnover timescale τ cv = 2π/ω cv (e.g.…”

“…The range of validity of the various turbulent viscosity prescriptions is shown in Figure 4, for binaries given in Lurie et al (2017). Both scalings are shown to be relevant for this sample.…”

“…This mechanism is usually invoked to explain the circularization and synchronization of binary systems containing low-mass or solar-like main-sequence stars (e.g. Zahn 1989;Meibom & Mathieu 2005;Meibom et al 2006;Van Eylen et al 2016;Lurie et al 2017;Triaud et al 2017;von Boetticher et al 2019), and evolved stars (e.g. Verbunt & Phinney 1995;Beck et al 2018;Price-Whelan & Goodman 2018).…”

“…Resolving this issue is essential before we can apply tidal theory to interpret observations of close binaries (e.g. Kirk et al 2016;Lurie et al 2017;Van Eylen et al 2016;Triaud et al 2017;Price-Whelan & Goodman 2018) or short-period planetary orbits (e.g. Rasio et al 1996).…”

Flare Activity of Wide Binary Stars with Kepler

“…The strength of the tidal force decreases as F r 1 T 3 µ , meaning an interaction with a third companion would have to occur at very close orbital separations in order to exert a sufficient tidal spin-up on the affected star. Lurie et al (2017) recently found in a sample of unresolved Kepler eclipsing binaries that tidal forces affected stellar rotation for systems with orbital periods shorter than 30 days, and typically led to orbit-spin synchronization for orbital periods shorter than 10 days. The faster rotators in the outlier pairs from our sample predominantly display P 10 rot < days.…”