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Evolutionary and structural models for contact binary stars make quantitative predictions about the distribution of systems in the mass ratio (q)–orbital period (P) plane. Specifically, contact binaries containing primaries with convective envelopes are predicted to be absent at mass ratios larger than a critical threshold that is a function of orbital period and total mass. We test this prediction by characterizing candidate contact binaries that appear to have mass ratios in violation of this threshold. We obtained quadrature-phase echelle spectra (R≈ 31,000) for 18 close binaries (0.65 day <P< 2.00 days) in the Kepler field, from which we extracted radial velocity profiles for each system. Use of a joint Markov Chain Monte Carlo fitting routine on the Kepler light curves and the radial velocity profiles allows us to retrieve all fundamental system and component parameters. Of the 18 systems, only one is a contact binary, and both components likely have radiative—not convective—envelopes. The 17 remaining systems are detached binaries (eight) or semidetached binaries (four) with ellipsoidal variations, rotating variables (four), or pulsating variables (one). Therefore, none of the systems are in violation of the theoretical mass ratio thresholds for low-mass contact binaries. The 12 noncontact binaries follow aT₂/T₁–qrelation significantly weaker than expected for main-sequence components, suggesting radiative heating of the secondaries. Most of the secondaries have radii larger than main-sequence expectations, a possible consequence of heating. Four secondaries fill their Roche lobes, while none of the primaries do, possibly indicating prior mass-ratio reversal.

Contact binary star systems represent the long-lived penultimate phase of binary evolution. Population statistics of their physical parameters inform an 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 days) tail of close binaries in the Kepler field. We use PHOEBE light-curve models to compute Bayesian probabilities on five principal system parameters. Mass ratios and third-light contributions measured from spectra agree well with those inferred from the light curves. Pilot study systems have extreme mass ratiosq< 0.32. Most are triples. Analysis of the unbiased sample of 783 0.15 d <P< 2 days (near) contact binaries results in 178 probable contact systems, 114 probable detached systems, and 491 ambiguous systems for which we report best-fitting and 16th-/50th-/84th-percentile parameters. Contact systems are rare at periodsP> 0.5 days, as are systems withq> 0.8. There exists an empirical mass ratio lower limit$q_{\min }(P)$≈ 0.05–0.15 below which contact systems are absent, supporting a new set of theoretical predictions obtained by modeling the evolution of contact systems under the constraints of mass and angular momentum conservation. Premerger systems should lie at long periods and near this mass ratio lower limit, which rises fromq= 0.044 forP= 0.74 days toq= 0.15 atP= 2.0 days. These findings support a scenario whereby nuclear evolution of the primary (more massive) star drives mass transfer to the primary, thus moving systems toward extremeqand largerPuntil the onset of the Darwin instability at$q_{\min }$precipitates a merger.

Transmission spectroscopy offers an invaluable opportunity to characterize the atmospheres of exoplanets. We present new ground-based optical transmission spectra of the hot Jupiter HD 189733b, derived from nine transits observed over a six year time span (2016–2021) using near-simultaneous$u\prime g\prime r\prime i\prime$broadband observations. We achieve an average (best) precision of 435 (280) ppm by implementing an optical diffuser on the prime focus spectrograph from the 2.3 m Wyoming Infrared Observatory telescope. The data provide new measurements of the apparent planetary radius with respect to the stellar radius, the spectral index of atmospheric opacity, and the time variability of the two quantities. Our results indicate an enhanced spectral slope in the optical regime ≈2.4 times steeper than would be expected from canonical Rayleigh scattering and that is consistent with earlier measurements of a super-Rayleigh slope (SRS). While the effect of stellar activity on the transmission spectrum complicates the measurement of the spectral slope, our multiepoch data set over six years can measure and average over stellar variations, yielding a mean spectral index of −9.9 ± 4.4. The 1200 K equilibrium temperature of HD 189733b places it in a sweet spot for the formation of SRSs and is consistent with vigorously mixing hazes in the atmosphere. Additionally, we find variations in the depth of the lightcurve during two of the transits, explainable as an increase in occulted star spots during June 2021. Although the star is active, the mean level of stellar activity does not seem to vary dramatically over our six years of observations, leading us to conclude that the variability in stellar activity is modest at most.