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

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 10μm. Here we suggest that the spectral data for HD 189733b are inconsistent with 8μm-photometry obtained with Spitzer ’s InfraRed Array Camera (IRAC), perhaps an indication of problems with the IRS spectra from 7.5-10μm. 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. Subject headings: planetary systems, radiative transfer, brown dwarfs, stars: individual (HD 189733, HD 209458) Space Science and Astrobiology Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035; [email protected], [email protected] Spitzer Fellow Carl Sagan Center, SETI Institute, 515 North Whisman Road, Mountain View, CA 94043