A radiative equilibrium model of 51 Peg b

We present a radiative equilibrium model for extrasolar giant planets applied to 51 Peg b. The atmospheric model extends from 10(-5) to similar to 10 bar and is limited at the bottom by an optically thick cloud of silicate (Mg2SiO4 or MgSiO3) or iron (Fe) particles. Rayleigh scattering at short wavelengths and absorption by the H-2-H-2 and H-2-He continuum and molecular bands of H2O, CO, and CH4 are included. Atmospheric heating and cooling result, respectively, from absorption of stellar flux and from infrared thermal emission. The solution temperature profiles do not show any temperature inversion, in contrast with the giant planets of the solar system. The lapse rate is subadiabatic at all levels above the cloudtop, justifying the use of radiative equilibrium. We find that, under thermochemical equilibrium, CO dominates over CH4 at all levels. The effective temperature is in the range 1150-1270 K and the Bond albedo in the range 0.15-0.42, depending on the location and reflectivity of the lower cloud deck. Mg2SiO4 or Fe clouds are weakly reflective in contrast to a MgSiO3 cloud. The thermal emission spectrum prevails over the stellar reflected component below 13,000-15,000 cm(-1) (lambda > 0.7 mu m); it shows various windows, between the H2O and CO bands, with the brightest centered at 2550 cm(-1) (3.9 mu m). The most prominent CO and CH4 bands occur around 2100 (4.7 mu m) and 3030 cm(-1) (3.3 mu m). Assuming a jovian abundance for PH3, the bands around 2300 (4.3 mu m) and 2450 cm(-1) (4.1 mu m) are clearly visible in absorption at a resolving power of similar to 100. We investigated the detectability of the minor species CO and CH4 at a resolution of 4000-6000, adequate to separate the planet's absorption features from their stellar and telluric counterparts. Considering photon noise from the star as the only noise source, we find that S/N ratios of about 8 are reached on the molecular features in the 4.7- and 3.3-mu m regions after similar to 10 h of integration on an 8-m telescope. On the other hand, the planet-to-star contrast is as low as (2-4) x 10(-4). (C) 2000 Academic Press.