Infrared Observations during the Secondary Eclipse of HD 209458b. I. 3.6 Micron Occultation Spectroscopy Using the Very Large Telescope

We search for an infrared signature of the transiting extrasolar planet HD209458b during secondary eclipse. Our method, which we call `occultationspectroscopy,' searches for the disappearance and reappearance of weak spectralfeatures due to the exoplanet as it passes behind the star and later reappears.We argue that at the longest infrared wavelengths, this technique becomespreferable to conventional `transit spectroscopy'. We observed the system inthe wing of the strong nu-3 band of methane near 3.6 microns during twosecondary eclipses, using the VLT/ISAAC spectrometer at a spectral resolutionof 3300. Our analysis, which utilizes a model template spectrum, achievessufficient precision to expect detection of the spectral structure predicted byan irradiated, low-opacity (cloudless), low-albedo, thermochemical equilibriummodel for the exoplanet atmosphere. However, our observations show no evidencefor the presence of this spectrum from the exoplanet, with the statisticalsignificance of the non-detection depending on the timing of the secondaryeclipse, which depends on the assumed value for the orbital eccentricity. Ourresults reject certain specific models of the atmosphere of HD 209458b asinconsistent with our observations at the 3-sigma level, given assumptionsabout the stellar and planetary parameters.Comment: 26 pages, 8 figures Accepted to Astrophysical Journa