Orbital Circularization of Hot and Cool Kepler Eclipsing Binaries

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 that include both hot and cool stars. Here we seek evidence of 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 ω based on the relative timing of the primary and secondary eclipses. We examine the distribution of e cos ω 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 four days, significant eccentricities occur frequently for hot-hot binaries, but not for hot-cool or coolcool 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.

EPIC 219388192b—An Inhabitant of the Brown Dwarf Desert in the Ruprecht 147 Open Cluster

“…because it is inversely proportional to q when  q 1 (e.g., Equation (2) in Van Eylen et al 2016). Therefore, it is not surprising to find such an eccentric binary system with a period of 5.29 days that is well below the circularization periods of coeval samples of stellar binaries because the latter have » q 1 (Meibom & Mathieu 2005;Milliman et al 2014).…”

Tidal Synchronization and Differential Rotation of Kepler Eclipsing Binaries

“…While numerous observational studies have focused on tidal circularization (e.g., Koch & Hrivnak 1981;Duquennoy & Mayor 1991;Meibom & Mathieu 2005;Van Eylen et al 2016), progress on tidal synchronization has been limited by three major factors. First, stellar rotation rates are generally more difficult to measure than orbital periods.…”

Dynamical Asteroseismology: towards improving the theories of stellar structure and (tidal) evolution

“…Although very interesting and valuable by itself, the study of [34] can be extended/improved on several aspects. First, this concerns including more massive and hotter stars into the sample so that it also covers the entire spectral type B and thus binaries with more extreme mass ratios.…”

“…The connection between the IGWs and the evolution of binary star systems in terms of circularisation of the orbit and synchronisation of the stellar components' spins are briefly touched upon in the next section. Figure 6: Top: A sample of some 900 Kepler binary systems on the e cos ω-log P diagram studied by [34]. The colours represent different system configurations in terms of the masses/spectral types of the individual stellar components: cool-cool (blue), hotcool (green), and hot-hot (red).…”

“…Recently, [34] investigated a sample of some 900 eclipsing binary stars observed by the original Kepler mission with the ultimate goal to confront the observations with the theoretical predictions concerning the circularisation time scales. Because of the lack of follow-up spectroscopic data, the parameter e cos ω was used as a proxy for the eccentricity of the system.…”