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Secondary eclipse observations of several of the hottest hot Jupiters showfeatureless, blackbody-like spectra or molecular emission features, which areconsistent with thermal inversions being present in those atmospheres. Theorypredicts a transition between warmer atmospheres with thermal inversions andcooler atmospheres without inversions, but the exact transition point isunknown. In order to further investigate this issue, we observed two secondaryeclipses of the hot Jupiter HAT-P-7b with the Hubble Space Telescope (HST) WFC3instrument and combined these data with previous Spitzer and Kepler secondaryeclipse observations. The HST and Spitzer data can be well fit by a blackbodywith $T=2692 \pm 14$ K, and the Kepler data point constrains the geometricalbedo to $A_{g}=0.077 \pm 0.006$. We modeled these data with a 3D GCM and 1Dself-consistent forward models. The 1D models indicate that the atmosphere hasa thermal inversion, weak heat redistribution, and water dissociation thatlimits the range of pressures probed. This result suggests that WFC3observations of HAT-P-7b and possibly some other ultra-hot Jupiters appearblackbody-like because they probe a region near the tropopause where theatmospheric temperature changes slowly with pressure. Additionally, the 1Dmodels constrain the atmospheric metallicity($[\text{M/H}]=-0.87^{+0.38}_{-0.34}$) and the carbon-to-oxygen ratio (C/O $<1$at 99 % confidence). The solar composition 3D GCM matches the Spitzer data butgenerally underpredicts the flux in the WFC3 bandpass and cannot reproduce itsfeatureless shape. This discrepancy could be explained by high atmospheric dragor nightside clouds, and may be better understood through further observationwith the James Webb Space Telescope (JWST).

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