An HST/WFC3 Thermal Emission Spectrum of the Hot Jupiter HAT-P-7b

Secondary eclipse observations ofseveral of the hottest hot Jupiters show featureless, blackbody-like spectra ormolecular emission features, which are consistent with thermal inversions beingpresent in those atmospheres. Theory predicts a transition between warmeratmospheres with thermal inversions and cooler atmospheres without inversions,but the exact transition point is unknown. In order to further investigate thisissue, we observed two secondary eclipses of the hot Jupiter HAT-P-7b with the Hubble Space Telescope (HST) WFC3 instrument and combined these data with previous Spitzer and Kepler secondary eclipse observations. The HST and Spitzerdata can be well fit by a blackbody with T = 2692 ± 14 K, and the Kepler data point constrains the geometric albedo to A g = 0.077 ± 0.006. We modeled thesedata with a three-dimensional (3D) GCM and one-dimensional (1D) self-consistentforward models. The 1D models indicate that the atmosphere has a thermalinversion, weak heat redistribution, and water dissociation that limits therange of pressures probed. This result suggests that WFC3 observations ofHAT-P-7b and possibly some other ultra-hot Jupiters appear blackbody-likebecause they probe a region near the tropopause where the atmospheric temperaturechanges slowly with pressure. Additionally, the 1D models constrain theatmospheric metallicity () and the carbon-to-oxygen ratio (C/O < 1 at 99%confidence). The solar composition 3D GCM matches the Spitzer data but generally underpredicts the flux in the WFC3bandpass and cannot reproduce its featureless shape. This discrepancy could beexplained by high atmospheric drag or nightside clouds and may be betterunderstood through further observation with the James Webb Space Telescope.