Water, Methane, and Carbon Dioxide Present in the Dayside Spectrum of the Exoplanet HD 209458b

Creators: Swain, M. R.; Tinetti, G.; Vasisht, G.; Deroo, P.; Griffith, C.; Bouwman, J.; Chen, Pin; Yung, Y.; Burrows, A. S.; Brown, L. R.; Matthews, J.; Rowe, J. F.; Kuschnig, R.; Angerhausen, D.

Abstract

Using the NICMOS instrument on the Hubble Space Telescope, we have measured the dayside spectrum of HD 209458b between 1.5 and 2.5 μm. The emergent spectrum is dominated by features due to the presence of methane (CH_4) and water vapor (H_2O), with smaller contributions from carbon dioxide (CO_2). Combining this near-infrared spectrum with existing mid-infrared measurements shows the existence of a temperature inversion and confirms the interpretation of previous photometry measurements. We find a family of plausible solutions for the molecular abundance and detailed temperature profile. Observationally resolving the ambiguity between abundance and temperature requires either (1) improved wavelength coverage or spectral resolution of the dayside emission spectrum or (2) a transmission spectrum where abundance determinations are less sensitive to the temperature structure.

Additional Information

© 2009 The American Astronomical Society. Received 2009 April 28; accepted 2009 August 21; published 2009 October 6. We appreciate the Director's Time Award for these observations, and we thank Tommy Wiklind, Beth Padillo, and other members of the Space Telescope Science Institute staff for assistance in planning the observations. We also thank Jonathan Tennyson and Bob Barber for help with the water line list. G.T. was supported by the Royal Society. A portion of the research described in this paper was carried out at the Jet Propulsion Laboratory, under a contact with the National Aeronautics and Space Administration.

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