Tidal evolution of circumbinary systems with arbitrary eccentricities: applications for Kepler systems

TL;DR

This paper extends an analytical model for tidal evolution in circumbinary systems with arbitrary eccentricities, applying it to Kepler systems to analyze their spin states, orbital migration, and eccentricity damping.

Contribution

The authors develop a self-consistent, high-order elliptical expansion model for tidal evolution that accurately handles arbitrary eccentricities and applies it to real Kepler circumbinary systems.

Findings

Kepler planets are likely in sub-synchronous spin states

All systems exhibit outward orbital migration due to tidal flow

Significant tidally induced eccentricity damping observed

Abstract

We present an extended version of the Constant Time Lag analytical approach for the tidal evolution of circumbinary planets introduced in our previous work. The model is self-consistent, in the sense that all tidal interactions between pairs are computed, regardless of their size. We derive analytical expressions for the variational equations governing the spin and orbital evolution, which are expressed as high-order elliptical expansions in the semimajor axis ratio but retain closed form in terms of the binary and planetary eccentricities. These are found to reproduce the results of the numerical simulations with arbitrary eccentricities very well, as well as reducing to our previous results in the low-eccentric case. Our model is then applied to the well-characterised Kepler circumbinary systems by analysing the tidal timescales and unveiling the tidal flow around each different system. In all cases we find that the spins reach stationary values much faster than the characteristic timescale of the orbital evolution, indicating that all Kepler circumbinary planets are expected to be in a sub-synchronous state. On the other hand, all systems are located in a tidal flow leading to outward migration; thus the proximity of the planets to the orbital instability limit may have been even greater in the past. Additionally, Kepler systems may have suffered a significant tidally induced eccentricity damping, which may be related to their proximity to the capture eccentricity. To help understand the predictions of our model, we also offer a simple geometrical interpretation of our results.