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C. Goukenleuque, B. B\'{e}zard, E. Lellouch (Obs. Paris, France)
We have developed a radiative equilibrium atmospheric model that we first applied to 51 Peg b orbiting 51 Peg A at 0.05 AU (Goukenleuque et al, Icarus, submitted). We present an extension of the model to other extrasolar systems with different masses and orbital distances of the planet.
In this model, the planet is in circular orbit and is externally heated by the stellar radiation. Opacity is provided by the rotational-vibrational bands of H2O, CO, CH4 and NH3, and by the collision-induced absorption of H2-H2 and H2-He pairs. The heating rates and cooling rates are calculated through a line-by-line multilayer radiative transfer code in the visible and IR wavelength range. The mean temperature profile is then derived, in the pressure range 10-5 to 1-30 bar, under the assumption of radiative equilibrium.
Simulations have been run for various masses (0.5-2 MJ) and orbital distances of the planet to the primary from 0.05 AU (51Peg b) to 0.23 AU (\rho CrB b). In all cases, the models do not show any temperature inversion. The thermal structure is subadiabatic at any pressure level, which validates the radiative equilibrium assumption. For 51 Peg b, we find that the model temperatures are in the range 700-1200 K above the 0.1-bar level, which is higher than predicted by Burrows et al.'s (1997) non gray model for isolated planets, but lower than Wiedemann's (1997) Jupiter-scaled model. At 0.05 AU, the CO/CH4 ratio is greater than 1 over the whole pressure grid, but it decreases as the 1-MJ planet orbits farther the primary. Finally, CH4 dominates over CO at all atmospheric levels beyond 0.23 AU.
For any planet, the emission and reflection spectra, from which Bond albedos and effective temperatures have been calculated, are dominated by the water vapor bands, revealing a pronounced atmospheric window at 4 \mum. We assessed the detectability of spectral signatures from close-in EGPs by by calculating high resolution spectra (R~5000) for 51 Peg b in the CH4 band at 3.3 \mum and CO band at 4.7 \mum.