Evidence for Atmospheric Cold-trap Processes in the Noninverted Emission Spectrum of Kepler-13Ab Using HST/WFC3
Authors:
Beatty et al
Abstract:
We observed two eclipses of the Kepler-13A planetary system, on UT 2014 April 28 and UT 2014 October 13, in the near-infrared using Wide Field Camera 3 on the Hubble Space Telescope. By using the nearby binary stars Kepler-13BC as a reference, we were able to create a differential light curve for Kepler-13A that had little of the systematics typically present in HST/WFC3 spectrophotometry. We measure a broadband (1.1–1.65 μm) eclipse depth of 734 ± 28 ppm and are able to measure the emission spectrum of the planet at R ≈ 50 with an average precision of 70 ppm. We find that Kepler-13Ab possesses a noninverted, monotonically decreasing vertical temperature profile. We exclude an isothermal profile and an inverted profile at more than 3σ. We also find that the dayside emission of Kepler-13Ab appears generally similar to an isolated M7 brown dwarf at a similar effective temperature. Due to the relatively high mass and surface gravity of Kepler-13Ab, we suggest that the apparent lack of an inversion is due to cold-trap processes in the planet's atmosphere. Using a toy model for where cold traps should inhibit inversions, as well as observations of other planets in this temperature range with measured emission spectra, we argue that with more detailed modeling and more observations we may be able to place useful constraints on the size of condensates on the daysides of hot Jupiters.
Spin-Orbit Angles of Kepler-13Ab and HAT-P-7b from Gravity-Darkened Transit Light Curves
Author:
Masuda
Abstract:
Analysis of the transit light curve deformed by the stellar gravity darkening allows us to photometrically measure both components of the spin-orbit angle ψ, its sky projection λ and inclination of the stellar spin axis i⋆. In this paper, we apply the method to two transiting hot Jupiter systems monitored with the Kepler spacecraft, Kepler-13A and HAT-P-7. For Kepler-13A, we find i⋆=81∘±5∘ and ψ=60∘±2∘ adopting the spectroscopic constraint λ=58.6∘±2.0∘ by Johnson et al. (2014). In our solution, the discrepancy between the above λ and that previously reported by Barnes et al. (2011) is solved by fitting both of the two parameters in the quadratic limb-darkening law. We also report the temporal variation in the orbital inclination of Kepler-13Ab, d|cosiorb|/dt=(−7.0±0.4)×10−6day−1, providing further evidence for the spin-orbit precession in this system. By fitting the precession model to the time series of iorb, λ, and i⋆ obtained with the gravity-darkened model, we constrain the stellar quadrupole moment J2=(6.1±0.3)×10−5 for our new solution, which is several times larger than J2=(1.66±0.08)×10−4 obtained for the previous one. We show that the difference can be observable in the future evolution of λ, thus providing a possibility to test our solution with follow-up observations. The second target, HAT-P-7, is the first F-dwarf star analyzed with the gravity-darkening method. Our analysis points to a nearly pole-on configuration with ψ=101∘±2∘ or 87∘±2∘ and the gravity-darkening exponent β consistent with 0.25. Such an observational constraint on β can be useful for testing the theory of gravity darkening.
A Misaligned Prograde Orbit for Kepler-13 Ab via Doppler Tomography
Authors:
Johnson et al
Abstract:
Transiting planets around rapidly rotating stars are not amenable to precise radial velocity observations, such as are used for planet candidate validation, as they have wide, rotationally broadened stellar lines. Such planets can, however, be observed using Doppler tomography, wherein the stellar absorption line profile distortions during transit are spectroscopically resolved. This allows the validation of transiting planet candidates and the measurement of the stellar spin-planetary orbit (mis)alignment, an important statistical probe of planetary migration processes. We present Doppler tomographic observations which provide a direct confirmation of the hot Jupiter Kepler-13 Ab, and also show that the planet has a prograde, misaligned orbit, with lambda = 58.6 +/- 2.0 degrees. Our measured value of the spin-orbit misalignment is in significant disagreement with the value of lambda = 23 +/- 4 degrees previously measured by Barnes et al. (2011) from the gravity-darkened Kepler lightcurve. We also place an upper limit of 0.75 solar masses (95% confidence) on the mass of Kepler-13 C, the spectroscopic companion to Kepler-13 B, the proper motion companion of the planet host star Kepler-13 A.
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.
