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

TL;DR

This study analyzes Spitzer mid-infrared spectra of irradiated exoplanets HD 209458b and HD 189733b, finding mostly featureless spectra and discussing potential issues with data reduction and model agreement, especially regarding water vapor detection.

Contribution

It provides a detailed comparison of observed spectra with atmospheric models, highlighting discrepancies and supporting the reliability of modeling approaches for irradiated planets.

Findings

Spectra are mostly featureless, lacking expected water vapor features.

IRAC photometry aligns with water absorption models, unlike IRS spectra.

Model spectra fit the data well without adjusting water abundance.

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 um. Here we suggest that, in the absence of flux variability, the spectral data for HD 189733b are inconsistent with 8 um-photometry obtained with Spitzer's InfraRed Array Camera (IRAC), perhaps an indication of problems with the challenging reduction of the IRS spectra. The IRAC point, along with previously published secondary eclipse photometry for HD 189733b, are in good agreement with a one-dimensional model of HD 189733b that clearly shows absorption due to water vapor in the emergent spectrum. We are not able to draw firm conclusions regarding the IRS data for HD 209458b, but spectra predicted by 1D and 3D atmosphere 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 lends confidence in the modeling procedure.