Orbital Circularization of Hot and Cool Kepler Eclipsing Binaries

TL;DR

This study analyzes Kepler eclipsing binaries to investigate how stellar type influences tidal circularization, finding that hot-hot binaries retain eccentricity longer at short periods, consistent with theoretical predictions.

Contribution

It provides observational evidence supporting the dependence of tidal circularization rates on stellar type using a large Kepler sample.

Findings

Hot-hot binaries are more eccentric at short periods.

Significant eccentricities are common for hot-hot binaries below 4 days.

Results align qualitatively with theoretical models of tidal dissipation.

Abstract

The rate of tidal circularization is predicted to be faster for relatively cool stars with convective outer layers, compared to hotter stars with radiative outer layers. Observing this effect is challenging, because it requires large and well-characterized samples including both hot and cool stars. Here we seek evidence for the predicted dependence of circularization upon stellar type, using a sample of 945 eclipsing binaries observed by Kepler. This sample complements earlier studies of this effect, which employed smaller samples of better-characterized stars. For each Kepler binary we measure $e\cos\omega$ based on the relative timing of the primary and secondary eclipses. We examine the distribution of $e\cos\omega$ as a function of period for binaries composed of hot stars, cool stars, and mixtures of the two types. At the shortest periods, hot-hot binaries are most likely to be eccentric; for periods shorter than 4 days, significant eccentricities occur frequently for hot-hot binaries, but not for hot-cool or cool-cool binaries. This is in qualitative agreement with theoretical expectations based on the slower dissipation rates of hot stars. However, the interpretation of our results is complicated by the largely unknown ages and evolutionary states of the stars in our sample.