Atmosphere and Spectral Models of the Kepler-Field Planets HAT-P-7b and TrES-2

TL;DR

This study models the atmospheres of Kepler-field exoplanets HAT-P-7b and TrES-2, revealing extreme thermal inversions and the impact of atmospheric chemistry on observed spectra, with implications for future observations.

Contribution

It provides the first detailed atmosphere models for these planets, including effects of nonequilibrium chemistry and predictions for Kepler-band flux.

Findings

HAT-P-7b requires an extremely hot upper atmosphere with a strong thermal inversion.

TrES-2 shows a mild thermal inversion with moderate day-night heat redistribution.

Predicted low secondary eclipse flux ratio (~2 x 10^-5) in Kepler bandpass for TrES-2.

Abstract

We develop atmosphere models of two of the three Kepler-field planets that were known prior to the start of the Kepler mission (HAT-P-7b and TrES-2). We find that published Kepler and Spitzer data for HAT-P-7b appear to require an extremely hot upper atmosphere on the dayside, with a strong thermal inversion and little day-night redistribution. The Spitzer data for TrES-2 suggest a mild thermal inversion with moderate day-night redistribution. We examine the effect of nonequilibrium chemistry on TrES-2 model atmospheres and find that methane levels must be adjusted by extreme amounts in order to cause even mild changes in atmospheric structure and emergent spectra. Our best-fit models to the Spitzer data for TrES-2 lead us to predict a low secondary eclipse planet-star flux ratio (~2 x 10^-5) in the Kepler bandpass, which is consistent with what very recent observations have found. Finally, we consider how the Kepler-band optical flux from a hot exoplanet depends on the strength of a possible extra optical absorber in the upper atmosphere. We find that the optical flux is not monotonic in optical opacity, and the non-monotonicity is greater for brighter, hotter stars.