Infrared observations during the secondary eclipse of HD 209458B. II. Strong limits on the infrared spectrum near 2.2 mu m

We report observations of the transiting extrasolar planet, HD 209458b, designed to detect the secondary eclipse. We employ the method of occultation spectroscopy,'' which searches in combined light ( star and planet) for the disappearance and reappearance of weak infrared spectral features due to the planet as it passes behind the star and reappears. Our observations cover two predicted secondary eclipse events, and we obtained 1036 individual spectra of the HD 209458 system using the SpeX instrument at the NASA Infrared Telescope Facility in 2001 September. Our spectra extend from 1.9 to 4.2 mum with a resolution (lambda/Deltalambda) of 1500. We have searched for a continuum peak near 2.2 mum (caused by CO and H2O absorption bands), as predicted by some models of the planetary atmosphere to be similar to 6 x 10(-4) of the stellar flux, but no such peak is detected at a level of similar to 3 x 10(-4) of the stellar flux. Our results represent the strongest limits on the infrared spectrum of the planet to date and carry significant implications for understanding the planetary atmosphere. In particular, some models that assume that the stellar irradiation is reradiated entirely on the substellar hemisphere predict a flux peak that is inconsistent with our observations. Several physical mechanisms can improve agreement with our observations, including the redistribution of heat by global circulation, a nearly isothermal atmosphere, and the presence of a high cloud.