Transmission Spectra as Diagnostics of Extrasolar Giant Planet Atmospheres

Atmospheres of transiting extrasolar giant planets (EGPs) such as HD 209458 bmust impose features on the spectra of their parent stars during transits;these features contain information about the physical conditions and chemicalcomposition of the atmospheres. The most convenient observational index showingthese features is the ``spectrum ratio'', defined as the wavelength-dependentratio of spectra taken in and out of transit. I describe a model that estimatesthis ratio and its dependence upon parameters of the planetary atmosphere,including its cloud structure, temperature, chemical composition, and windfields. For giant planets in close orbits, the depths of atomic and molecularfeatures in the spectrum ratio may be as large as 0.001. Observations invisible and near-IR wavelengths using existing and planned spectrographs shouldbe adequate to detect these features, and to provide some diagnostics of theconditions within the planetary atmosphere. I give numerous examples of suchdiagnostics, and I discuss their practicality.Comment: 57 pages, 22 figures. To be published in the Astrophysical Journa