Analysis of Spitzer 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 shortward of . Here we suggest that in the absence of flux variability, the spectral data for HD 189733b 10 mm are inconsistent with 8 mm 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. Subject headings: planetary systems — radiative transfer — stars: individual (HD 189733, HD 209458)

[1]  Drake Deming,et al.  Infrared radiation from an extrasolar planet , 2005, Nature.

[2]  M. Mayor,et al.  A Jupiter-mass companion to a solar-type star , 1995, Nature.

[3]  D. A. Golimowski,et al.  L' AND M' Photometry Of Ultracool Dwarfs , 2004 .

[4]  J. R. Houck,et al.  A Spitzer Infrared Spectrograph Spectral Sequence of M, L, and T Dwarfs , 2006 .

[5]  J. S. Hogan,et al.  Radiative-convective equilibrium models of Jupiter and Saturn , 1984 .

[6]  C. McKay,et al.  The thermal structure of Titan's atmosphere. , 1989, Icarus.

[7]  T. Barman.  Identification of Absorption Features in an Extrasolar Planet Atmosphere , 2007, 0704.1114.

[8]  D. Saumon,et al.  3.6-7.9 μm Photometry of L and T Dwarfs and the Prevalence of Vertical Mixing in their Atmospheres , 2006, astro-ph/0610214.

[9]  Sara Seager,et al.  “Weather” Variability of Close-in Extrasolar Giant Planets , 2002, astro-ph/0210499.

[10]  A. Burrows,et al.  A Spitzer Spectrum of the Exoplanet HD 189733b , 2007 .

[11]  A time-dependent radiative model of HD 209458b , 2004, astro-ph/0409468.

[12]  C P McKay,et al.  Thermal structure of Uranus' atmosphere. , 1999, Icarus.

[13]  G. Fazio,et al.  The Infrared Array Camera (IRAC) for the Spitzer Space Telescope , 2004, astro-ph/0405616.

[14]  Spitzer Infrared Spectrograph Observations of L, M, and T Dwarfs , 2004 .

[15]  R. Kurucz.  ATLAS9 Stellar Atmosphere Programs and 2 km/s grid. , 1993 .

[16]  S. Seager,et al.  Clouds and chemistry: Ultracool dwarf atmospheric properties from optical and infrared colors , 2002 .

[17]  Curtis S. Cooper,et al.  Dynamics and Disequilibrium Carbon Chemistry in Hot Jupiter Atmospheres, with Application to HD 209458b , 2006 .

[18]  C. McKay,et al.  Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres , 1989 .

[19]  et al,et al.  Near-Infrared Photometry and Spectroscopy of L and T Dwarfs: The Effects of Temperature, Clouds, and Gravity , 2004, astro-ph/0402451.

[20]  David Charbonneau,et al.  Using Stellar Limb-Darkening to Refine the Properties of HD 209458b , 2006, astro-ph/0603542.

[21]  A. P. Showman,et al.  The Influence of Atmospheric Dynamics on the Infrared Spectra and Light Curves of Hot Jupiters , 2006 .

[22]  L. J. Richardson,et al.  On the Dayside Thermal Emission of Hot Jupiters , 2005 .

[23]  H. Maring,et al.  Journal of Geophysical Research , 1949, Nature.

[24]  David Charbonneau,et al.  A map of the day–night contrast of the extrasolar planet HD 189733b , 2007, Nature.

[25]  John W. Mason.  Astrophysics update 2 , 2006 .

[26]  M. Cushing,et al.  Spitzer Infrared Spectrograph (IRS) Observations of M, L, and T Dwarfs , 2004 .

[27]  Avi Shporer,et al.  The Transit Light Curve Project. V. System Parameters and Stellar Rotation Period of HD 189733 , 2006, astro-ph/0612224.

[28]  J. Lunine,et al.  Reflected Spectra and Albedos of Extrasolar Giant Planets. I. Clear and Cloudy Atmospheres , 1998, astro-ph/9810073.

[29]  B. Fegley,et al.  Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars: I. Carbon, Nitrogen, and Oxygen , 2002 .

[30]  Comparative Planetary Atmospheres: Models of TrES-1 and HD 209458b , 2005, astro-ph/0505359.

[31]  M. Hanner,et al.  Silicate Emission Feature in the Spectrum of Comet Mueller 1993a , 1994 .

[32]  John C. Wilson,et al.  Moderate-Resolution Spitzer Infrared Spectrograph Observations of M, L, and T Dwarfs , 2007, astro-ph/0701398.

[33]  Drake Deming,et al.  Accepted for publication in the Astrophysical Journal Strong Infrared Emission from the Extrasolar Planet HD189733b , 2006 .

[34]  I. Hubeny,et al.  Theory for the Secondary Eclipse Fluxes, Spectra, Atmospheres, and Light Curves of Transiting Extrasolar Giant Planets , 2006, astro-ph/0607014.

[35]  Drake Deming,et al.  A spectrum of an extrasolar planet , 2007, Nature.

[36]  D. Saumon,et al.  Atmosphere, Interior, and Evolution of the Metal-rich Transiting Planet HD 149026b , 2006 .

[37]  Sara Seager,et al.  Hot Jupiter Variability in Eclipse Depth , 2006, astro-ph/0612413.

[38]  Ammonia as a tracer of chemical equilibrium in the T7.5 dwarf Gliese 570D , 2006, astro-ph/0605563.

[39]  K. Lodders.  Solar System Abundances and Condensation Temperatures of the Elements , 2003 .