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.
Highlights
Observations of the emergent spectra from transiting extrasolar planets with the Spitzer Space Telescope have enabled us to probe the atmospheres of a class of giant extrasolar planets known as “hot Jupiters.” These planets have masses and radii similar to the gas giants in our solar system, but orbit very close to their parent stars, with equilibrium temperatures ranging from 1000 to 2500 K
We observed a secondary eclipse of WASP-4b in the 4.5 μm band on UT 2009 December 6 using Infrared Array Camera (IRAC) on board the Spitzer Space Telescope
By measuring the time of the eclipse, we estimate a 2σ upper limit on the parameter |e cos ω| of 0.0024. This limit implies that unless our line of sight happens to align closely to the planet’s major axis, the planet’s orbit must be nearly circular. This upper limit does not rule out tidal heating, it constrains the range of tidal heating models that could explain this planet’s inflated radius
Summary
Observations of the emergent spectra from transiting extrasolar planets with the Spitzer Space Telescope have enabled us to probe the atmospheres of a class of giant extrasolar planets known as “hot Jupiters.” These planets have masses and radii similar to the gas giants in our solar system, but orbit very close to their parent stars, with equilibrium temperatures ranging from 1000 to 2500 K. Observations of the emergent spectra from transiting extrasolar planets with the Spitzer Space Telescope have enabled us to probe the atmospheres of a class of giant extrasolar planets known as “hot Jupiters.” These planets have masses and radii similar to the gas giants in our solar system, but orbit very close to their parent stars, with equilibrium temperatures ranging from 1000 to 2500 K. Spitzer obtained multi-wavelength observations for 15 extrasolar planets during secondary eclipse The results of these studies indicate that hot Jupiter atmospheres can be distinguished by the presence or absence of a strong temperature inversion in the upper atmosphere (e.g., Burrows et al 2007b, 2008; Fortney et al 2008; Barman 2008; Madhusudhan et al 2009).
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