Secondary Eclipse Photometry of WASP-4b with Warm Spitzer
We present photometry of the giant extrasolar planet WASP-4b at 3.6 and 4.5 μm 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 μm 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. We also find no evidence for an offset in the timing of the secondary eclipse and place a 2σ upper limit on |ecos ω| of 0.0024, which constrains the range of tidal heating models that could explain this planet's inflated radius.
Additional Information© 2011 American Astronomical Society. Received 2010 July 22; accepted 2010 November 8; published 2010 December 28. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Support for this work was provided by NASA. Heather A. Knutson is supported by a fellowship from the Miller Institute for Basic Research Science. Eric Agol acknowledges the support of NSF CAREER grant No. 0645416.
Published - 0004-637X_727_1_23.pdf
Accepted Version - 1011.4066.pdf