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We report observations of the transiting extrasolar planet, HD 209458 b,designed to detect the secondary eclipse. We employ the method of `occultationspectroscopy', which searches in combined light (star and planet) for thedisappearance and reappearance of weak infrared spectral features due to theplanet as it passes behind the star and reappears. Our observations cover twopredicted secondary eclipse events, and we obtained 1036 individual spectra ofthe HD 209458 system using the SpeX instrument at the NASA IRTF in September2001. Our spectra extend from 1.9 to 4.2 microns with a spectral resolution of1500. We have searched for a continuum peak near 2.2 microns (caused by CO andwater absorption bands), as predicted by some models of the planetaryatmosphere to be approximately 6E-4 of the stellar flux, but no such peak isdetected at a level of about 3E-4 of the stellar flux. Our results representthe strongest limits on the infrared spectrum of the planet to date and carrysignificant implications for understanding the planetary atmosphere. Inparticular, some models that assume the stellar irradiation is re-radiatedentirely on the sub-stellar hemisphere predict a flux peak inconsistent withour observations. Several physical mechanisms can improve agreement with ourobservations, including the re-distribution of heat by global circulation, anearly isothermal atmosphere, and/or the presence of a high cloud.Comment: Accepted to the Astrophysical Journal 17 pages, 6 figure

Infrared Observations During the Secondary Eclipse of HD 209458b. II. Strong Limits on the Infrared Spectrum Near 2.2 μm