Mass-ratio distribution of contact binary stars

Abstract

The mass ratio $q$ of a contact binary star evolves due to mass transfer, magnetic braking, and thermal relaxation oscillations to small values until it crosses a critical threshold $q_{\text{min}}$. When that happens, the binary undergoes the tidal Darwin instability, leading to a rapid coalescence of the components and observable brightening of the system. So far, the distribution of $q$ has not been measured on a sufficiently large population of contact binary stars, because the determination of $q$ for a single contact binary usually requires spectroscopy. But as was shown previously, it is possible to infer the mass-ratio distribution of the entire population of contact binaries from the observed distribution of their light curve amplitudes. Employing Bayesian inference, we obtain a sample of contact binary candidates from the Kepler Eclipsing Binary Catalog combined with data from Gaia and estimates of effective temperatures. We assign to each candidate a probability of being a contact binary of either late or early type. Overall, our sample includes about 300 late-type and 200 early-type contact binary candidates. We model the amplitude distribution assuming that mass ratios are described by a power law with an exponent $b$ and a cut off at $q_{\text{min}}$. We find $q_{\text{min}}=0.087_{-0.015}^{+0.024}$ for late-type contact binaries with periods longer than 0.3 days. For late-type binaries with shorter periods, we find $q_{\text{min}}=0.246_{-0.046}^{+0.029}$, but the sample is small. For early type contact binary stars with periods shorter than 1 day, we obtain $q_{\text{min}}=0.030_{-0.022}^{+0.018}$. These results indicate a dependence of $q_{\text{min}}$ on the structure of the components and are broadly compatible with previous theoretical predictions. Our method can be easily extended to large samples of contact binaries from TESS and other space-based surveys.

Accepted to A&A. 28 pages, 22 figures, 4 tables

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