Secondary Eclipse Photometry of WASP-4b with Warm Spitzer

TL;DR

This study measures the secondary eclipse depths of exoplanet WASP-4b at infrared wavelengths, analyzes its atmospheric composition and temperature structure, and investigates implications for atmospheric inversions and planetary heating.

Contribution

First infrared secondary eclipse measurements of WASP-4b with detailed atmospheric modeling and implications for atmospheric inversions and planetary heating mechanisms.

Findings

WASP-4b shows no strong thermal inversion, contrary to expectations for highly irradiated planets.

Eclipse depths are consistent with models containing water and other molecules in absorption.

No significant eclipse timing offset was detected, constraining orbital and tidal heating models.

Abstract

We present photometry of the giant extrasolar planet WASP-4b at 3.6 and 4.5 micron taken with the Infrared Array Camera on board the Spitzer Space Telescope as part of Spitzer's extended warm mission. We find secondary eclipse depths of 0.319+/-0.031% and 0.343+/-0.027% for the 3.6 and 4.5 micron bands, respectively and show model emission spectra and pressure-temperature profiles for the planetary atmosphere. These eclipse depths are well fit by model emission spectra with water and other molecules in absorption, similar to those used for TrES-3 and HD 189733b. Depending on our choice of model, these results indicate that this planet has either a weak dayside temperature inversion or no inversion at all. The absence of a strong thermal inversion on this highly irradiated planet is contrary to the idea that highly irradiated planets are expected to have inversions, perhaps due the presence of an unknown absorber in the upper atmosphere. This result might be explained by the modestly enhanced activity level of WASP-4b's G7V host star, which could increase the amount of UV flux received by the planet, therefore reducing the abundance of the unknown stratospheric absorber in the planetary atmosphere as suggested in Knutson et al. (2010). We also find no evidence for an offset in the timing of the secondary eclipse and place a 2 sigma upper limit on |ecos(omega)| of 0.0024, which constrains the range of tidal heating models that could explain this planet's inflated radius.