Analysis of Spitzer Mid Infrared Spectra of Irradiated Planets: Evidence for Water Vapor?

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Abstract
Published mid infrared spectra of transiting planets HD 209458b and HD 189733b, obtained during secondary eclipse by the InfraRed Spectrograph (IRS) aboard the Spitzer Space Telescope, are predominantly featureless. In particular these flux ratio spectra do not exhibit an expected feature arising from water vapor absorption short-ward of 10 microns. Here we suggest that the spectral data for HD 189733b are inconsistent with 8 micron-photometry obtained with Spitzer's InfraRed Array Camera (IRAC), perhaps an indication of problems with the IRS spectra from 7.5-10 microns. The IRAC data, along with previously published secondary eclipse photometry for HD 189733b, are in excellent agreement with a one-dimensional model of HD 189733b that clearly predicts water absorption. We are not able to draw firm conclusions regarding the IRS data for HD 209458b, but spectra predicted by 1D and 3D atmospheres models fit the data adequately, without adjustment of the water abundance or reliance on cloud opacity. We argue that the generally good agreement between model spectra and IRS spectra of brown dwarfs with atmospheric temperatures similar to these highly irradiated planets support the veracity of the modeling procedure.

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