Discovery of Six Optical Phase Curves with K2

Abstract

We have systematically searched for the phase curves among the planets discovered by K2. Using the reported planetary parameters, we screen out the best potential candidates, and examine their light curves in detail. For our work, we consider light curves from two different detrending pipelines -EVEREST and K2SFF. In order to remove stellar variability and systematics, we test three different filtering techniques: spline, phasma (median-filtering) and Butterworth (harmonics filtering), and use Butterworth filtered light curves for the subsequent analysis. We have identified 6 previously unreported phase curves among the planets observed with K2 : K2-31b, HATS-9b, HATS-11b, K2-107b, K2-131b, and K2-106b. The first four of these are hot Jupiters for which we find the photometric masses consistent with their RV-based masses within 2σ, 1σ, 1σ, and 3σ respectively with comparatively low geometric albedos, while the last two are ultra-short period super-Earths with phase curves dominated by reflective and thermal components. We also detect a secondary eclipse in HATS-11b at 62±12 ppm. We thus deem it to be possible to validate the planetary nature of selected K2, and suggest similar vetting could be used for the ongoing TESS mission.

Spatially Resolved Modeling of Optical Albedos for a Sample of Six Hot Jupiters

“…Parmentier et al 2016;Powell et al 2018;Parmentier et al 2021;Gao et al 2020;Roman et al 2021). This temperature range contains some of the most reflective hot Jupiters observed to date, including Kepler-7b (Demory et al 2013;Heng et al 2021) and HATS-11b (Niraula et al 2018). Note that two observed geometric albedos have been reported for Kepler-7b, and from here on we consider the most recent value from Heng et al (2021).…”

“…Note that two observed geometric albedos have been reported for Kepler-7b, and from here on we consider the most recent value from Heng et al (2021). It also includes three moderately reflective hot Jupiters (Kepler-8b, Kepler-17b, and K2-107b; Esteves et al 2013;Desert et al 2011;Niraula et al 2018) and one very dark hot Jupiter (K2-31b; Niraula et al 2018). Previous studies (e.g., Demory et al 2013) have concluded that planets with these equilibrium temperatures should have negligible amounts of thermal emission in the Kepler bandpass.…”

“…Furthermore, there is direct observational evidence for inhomogeneous cloud structures stemming from reflected-light phase curves (e.g., Demory et al 2013). To date, three of the four planets with observed reflected-light phase curves appear to have patchy clouds (Desert et al 2011;Demory et al 2013;Angerhausen et al 2014;Esteves et al 2015;Shporer & Hu 2015;von Paris et al 2016;Niraula et al 2018, also see Figure 1). Kepler-7b, -12b, and -41b all appear to have patchy clouds that are preferentially concentrated in the western (dawn) hemisphere, causing the peak of the phase curve to occur after the secondary eclipse (Shporer & Hu 2015).…”

“…Most of these albedo measurements have come from space telescopes observing in broad optical bandpasses (e.g. Rowe et al 2008;Kipping & Bakos 2011;Demory et al 2011Demory et al , 2013Coughlin & Lopez-Morales 2012;Parviainen et al 2013;Angerhausen et al 2014;von Paris et al 2016;Niraula et al 2018). The hot Jupiter HD 189733b is currently the only planet with a spectroscopically resolved reflected light measurement (Evans et al 2013, geometric albedo of 0.40 ± 0.12 at 290 − 450 nm and < 0.12 at 450 − 570 nm).…”

“…Cloud-free atmosphere models for hot Jupiters generally predict dayside albedos of less than 0.1 (Seager et al 2000), but the observed optical geometric albedo measurements published to date span a wide range of values, with the brightest planets exhibiting albedos as high as 0.3 (e.g. Heng & Demory 2013;Niraula et al 2018). In the solar system, the presence of water clouds increases Earth's geometric albedo to approximately 0.37 (e.g.…”