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Published February 10, 2008 | Published
Journal Article Open

The mid-infrared spectrum of the transiting exoplanet HD 209458b


We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b. Using archive data taken with the Spitzer IRS instrument, we have determined the spectrum of HD 209458b between 7.46 and 15.25 μm. We have used two independent methods to determine the planet spectrum, one differential in wavelength and one absolute, and find the results are in good agreement. Over much of this spectral range, the planet spectrum is consistent with featureless thermal emission. Between 7.5 and 8.5 μm, we find evidence for an unidentified spectral feature. If this spectral modulation is due to absorption, it implies that the dayside vertical temperature profile of the planetary atmosphere is not entirely isothermal. Using the IRS data, we have determined the broadband eclipse depth to be 0.00315 ± 0.000315, implying significant redistribution of heat from the dayside to the nightside. This work required the development of improved methods for Spitzer IRS data calibration that increase the achievable absolute calibration precision and dynamic range for observations of bright point sources.

Additional Information

© 2008 The American Astronomical Society. Received 2007 January 17; accepted 2007 September 18. We thank the original PI team for the proposal and preparation of the AORs required to obtain these data. We appreciate the comments of the anonymous referee, who encouraged us to fully describe our calibration process and extend our calibration method to the entire IRS SL1 passband; these suggestions led to the detection of spectral modulation that was outside our original spectral passband. We also thank Drake Deming for several helpful conversations regarding the reduction of secondary eclipse data. We thank John Bayard for several helpful discussions concerning Spitzer pointing errors and Sara Seager for a discussion regarding the possible role of clouds in exoplanet atmospheres.

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