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J. Harrington (Cornell), L. D. Deming (NASA/GSFC), K. Matthews (Caltech), L. J. Richardson (U. Colorado, Boulder), P. Rojo (Cornell), D. Steyert (NASA/GSFC), G. Wiedemann (Uni. Sternwarte Jena), D. Zeehandelaar (Cornell)
We search for the spectral signature of H2O, CH4, and CO on the extrasolar planet HD 209458b using transit spectroscopy. The planet will modulate the stellar spectrum during transit. The extinction altitude of tangent rays of a given wavelength depends on the abundance and distribution of molecular species that absorb at that wavelength. The depth of the occultation at a given wavelength depends on the cross-sectional area of the planet, which is determined by the extinction altitude. For deep lines of H2O, CH4, and CO, the effect is about 0.07%, due to the ~750 km scale height. Our S/N calculations show that well-calibrated, ground-based spectra, integrated over time and wavelength, can measure or place useful limits on the atmospheric abundances of H2O, CH4, and CO. Carbon forms predominantly CH4 below 1400 K and CO if hotter. Since the effective temperature is around 1400 K, detecting CH4 and/or CO would provide a tight constraint on atmospheric temperatures. Observations began in August 2001 at Palomar and we have 12 additional nights granted in 2002 at Keck, VLT, and IRTF. The expected modulation of the stellar spectrum is model-dependent: high-altitude hazes, photochemical effects, and different thermal profiles would substantially modify the effect for the same planetary elemental abundances. Since the effect is subtle compared to the noise in the data, we correlate model spectra against thousands of observed spectra and average the correlations to test whether the data support a given model. We are developing a radiative-transfer model to predict the spectrum of a given planetary model, and we are measuring H2O, CH4, and CO in the laboratory at 1300 K, with pressure-broadening by H2, to make our model spectra realistic at these elevated temperatures (crucial for detecting H2O). Analysis of the 2001 data and acquisition of 2002 data are in progress. We solicit participation by those who wish to test their planetary models.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.