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Creators/Authors contains: "Cook, Evan M."

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  1. Abstract

    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 limitqmin(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 shouldmore »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 atqminprecipitates a merger.

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  2. Abstract 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 ′ g ′ r ′ i ′ 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 ofmore »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.« less
    Free, publicly-accessible full text available December 6, 2023