http://arxiv.org/abs/2202.01187
Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform understanding of binary evolutionary pathways and end products. We use light curves and new optical spectroscopy to conduct a pilot study of ten (near-)contact systems in the long-period (P>0.5 d) tail of close binaries in the Kepler field. We couple PHOEBE light curve models with Markov-Chain Monte Carlo analyses to compute Bayesian probabilities on inclinations, fillout factors, mass ratios, third-light fractions, and component temperature ratios. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the light curves. Most binaries in the pilot study have extreme mass ratios q<0.32. At least eight are probable triples. A Bayesian analysis of all 783 Kepler 0.15 d<P<2 d (near-)contact binaries forming an unbiased sample of unprecedented size and photometric precision results in 178 probable contact systems, 114 probable detached systems, and 491 ambiguous systems. We report best-fitting and 16th/50th/84th percentile parameters of modeled systems. Contact systems are rare at periods P>0.5 d, as are systems with mass ratios near unity (q>0.8). There exists an empirical mass ratio lower limit $q_{min}$(P)~0.05-0.15 below which contact systems are absent, in agreement with a new set of theoretical predictions we obtain by modeling the evolution of contact systems under the constraints of mass and angular momentum conservation. Pre-merger systems should be found at at long periods and near this mass ratio lower limit, which rises from q=0.044 for P=0.74 d to q=0.15 at P=2.0 d. These findings support a scenario whereby mass transfer drives systems toward extreme $q$ and larger $P$ until the onset of the Darwin instability at $q_{min}$ precipitates a merger.
H. Kobulnicky, L. Molnar, E. Cook, et. al.
Thu, 3 Feb 22
29/56
Comments: Resubmitted to ApJ after revisions following reviewer comments
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