A chemical model for the atmosphere of hot Jupiters

Abstract

Context. The atmosphere of hot Jupiters can be probed by primary transit and secondary eclipse spectroscopy. Owing to the intense UV irradiation, mixing, and circulation, their chemical composition is maintained out of equilibrium and must be modeled with kinetic models. Aims. Our purpose is to release a chemical network and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD 209458b and HD 189733b) with a model that includes photolyses and vertical mixing, and we produce synthetic spectra. Methods. The chemical scheme has been derived from applied combustion models that were methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters. We compared the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species, and with previously published photochemical models. Results. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever nonequilibrium processes take place (photodissociations or vertical mixing). The deviations from the equilibrium, hence the sensitivity to the network, are larger for HD 189733b, since we assume a cooler atmosphere than for HD 209458b. We found that the abundances of NH 3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH 3 at 10.5 μm is sensitive to these abundance variations and thus to the chemical scheme. Conclusions. Due to the influence of the kinetics, we recommend using a validated scheme to model the chemistry of exoplanet atmospheres. The network we release is robust for temperatures within 300-2500 K and pressures from 10 mbar up to a few hundred bars, for species made of C, H, O, and N. It is validated for species up to 2 carbon atoms and for the main nitrogen species (NH 3, HCN, N 2, NO x). Although the influence of the kinetic scheme on the hot Jupiters spectra remains within the current observational error bars (with the exception of NH 3), it will become more important for atmospheres that are cooler or subjected to higher UV fluxes, because they depart more from equilibrium. ©2012 ESO.