Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data

We present theoretical atmosphere, spectral, and light-curve models for extrasolar giant planets (EGPs) undergoing strong irradiation for which Spitzer planet/star contrast ratios or light curves have been published (circa 2007 June). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD 179949b, and υ And b. By comparing models with data, we find that a number of EGP atmospheres experience thermal inversions and have stratospheres. This is particularly true for HD 209458b, HD 149026b, and υ And b. This finding translates into qualitative changes in the planet/star contrast ratios at secondary eclipse and in close-in EGP orbital light curves. Moreover, the presence of atmospheric water in abundance is fully consistent with all the Spitzer data for the measured planets. For planets with stratospheres, water absorption features invert into emission features and mid-infrared fluxes can be enhanced by a factor of 2. In addition, the character of near-infrared planetary spectra can be radically altered. We derive a correlation between the importance of such stratospheres and the stellar flux on the planet, suggesting that close-in EGPs bifurcate into two groups: those with and without stratospheres. From the finding that TrES-1 shows no signs of a stratosphere, while HD 209458b does, we estimate the magnitude of this stellar flux breakpoint. We find that the heat redistribution parameter, Pn, for the family of close-in EGPs assumes values from ~0.1 to ~0.4. This paper provides a broad theoretical context for the future direct characterization of EGPs in tight orbits around their illuminating stars.

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