The Anticorrelated Nature of the Primary and Secondary Eclipse Timing Variations for the Kepler Contact Binaries

TL;DR

This study analyzes eclipse timing variations in Kepler contact binaries, revealing anticorrelated primary and secondary eclipse shifts caused by starspots, challenging mass transfer explanations and linking observed quasi-periodicities to stellar differential rotation.

Contribution

It provides the first detailed analysis of anticorrelated eclipse timing variations in contact binaries, attributing them to starspots rather than mass transfer, and estimates differential rotation parameters.

Findings

Approximately 390 binaries show anticorrelated O-C variations.

Starspots can explain the observed anticorrelations and amplitudes.

Quasi-periodicities suggest specific differential rotation coefficients.

Abstract

We report on a study of eclipse timing variations in contact binary systems, using long-cadence lightcurves in the Kepler archive. As a first step, 'observed minus calculated' (O-C) curves were produced for both the primary and secondary eclipses of some 2000 Kepler binaries. We find ~390 short-period binaries with O-C curves that exhibit (i) random-walk like variations or quasi-periodicities, with typical amplitudes of +/- 200-300 seconds, and (ii) anticorrelations between the primary and secondary eclipse timing variations. We present a detailed analysis and results for 32 of these binaries with orbital periods in the range of 0.35 +/- 0.05 days. The anticorrelations observed in their O-C curves cannot be explained by a model involving mass transfer, which among other things requires implausibly high rates of ~0.01 M_sun per year. We show that the anticorrelated behavior, the amplitude of the O-C delays, and the overall random-walk like behavior can be explained by the presence of a starspot that is continuously visible around the orbit and slowly changes its longitude on timescales of weeks to months. The quasi-periods of ~50-200 days observed in the O-C curves suggest values for k, the coefficient of the latitude dependence of the stellar differential rotation, of ~0.003-0.013.