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 close-in extrasolar giant planets (EGPs) undergoing strong stellar irradiation for which {\it Spitzer} planet/star contrast ratios or light curves have been published (circa June 2007). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD 179949b, and $\upsilon$ And b. By comparing our models with these data, we find that some EGP atmospheres experience thermal inversions and have stratospheres. This is particularly true for HD 209458b, HD 149026b, and $\upsilon$ 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 {\it 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 as much as a factor of two. 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. Furthermore, we find that the heat redistribution parameter, P$_n$, for the family of close-in EGPs assumes values from $\sim$0.1 to $\sim$0.4. However, our current constraints on this parameter are rather weak. This paper is meant in part to provide a broad theoretical context for the future direct characterization of EGPs in tight orbits around their illuminating stars.