Characterizing Hot Jupiter Kepler-13Ab's Exoatmosphere

Atmospheric Characterization of the Hot Jupiter Kepler-13Ab

Authors:

Shporer et al

Abstract:

Kepler-13Ab (= KOI-13.01) is one of very few known short-period (1.76 day) transiting planets orbiting a bright A-type star. The availability of Kepler data allows a measurement of the planet's occultation (secondary eclipse) and orbital phase curve, which we combine with occultations observed by Spitzer at 3.6 micron and 4.5 micron and a ground-based occultation observation in the Ks band (2.1 micron). For the day-side hemisphere we derive a temperate of 2,750 ± 160 K as the effective temperature of a black body that will show the same occultation depths, and a high geometric albedo Ag = 0.33+0.04−0.06. Comparing the occultation depths with one-dimensional planetary atmosphere models suggests the presence of an atmospheric temperature inversion. The Kepler mid-occultation time measured here is 34.0 ± 6.9 s earlier than expected based on the mid-transit time reported in the literature and the expected time delay due to light travel time. This could be due to a small orbital eccentricity or asymmetric planetary surface brightness distribution. The planet host star is fully blended in all our photometric data with a visual binary companion, which is another A-type star 1.1" away that itself has a late-type stellar binary companion. To correct the dilution in our measured occultation depths we modeled the flux ratio between the two stars based on Keck/HIRES spectra where the two stars are spatially resolved. We used these spectra also to revise the stellar parameters for the planet host star. These parameters, combined with the photometric amplitudes of the beaming effect and the tidal ellipsoidal distortion measured here in the Kepler phase curve lead to a revised planetary mass estimate of Mp = 4.94 - 8.09 Mj, and combined with the planet to star radii ratio reported in the literature lead to a revised planet radius of Rp = 1.406 ± 0.038 Rj.