Photochimie des exoplanètes chaudes: modélisations et expériences

Hot Jupiters are a class of exoplanets very interesting to study. Indeed, these giant planets, orbiting very close to their star (typically 0.05 AU), receive a UV flux 10 000 times more intense that the one Jupiter receives in our Solar System. The atmospheric temperature, thus very high, ranges between 1000 and 3000 K. Because of these high temperatures, the important dynamic and strong UV irradiation, the atmospheres of these planets are the site of a unique chemistry, having no equivalent in the Solar System. In addition, some hot Jupiters are relatively close to us and pass in front of their star. Thus, we can make spectra that give us information on the composition and thermal structure of their atmospheres. Given our current technology, they are the only exoplanets for which it is possible to obtain such measures. During this thesis, we have developed a 1D photochemical model to study the atmosphere of hot Jupiters. This model, including vertical transport and photodissociations, is based on an inedit chemical scheme in planetology. The chemical network has been developed in collaboration with a specialist of combustion and was experimentally validated over a wide pressure range (10-5 - 100 bar) and temperature (300 - 2500 K). This network, describing the kinetics of the species up to C2 (species with two carbon atoms) has been made available to the scientific community on the database KIDA. This robust chemical scheme allows us to be highly confident in the results provided by our model because it is the first network validated for the atmospheric conditions studied. We have modeled the atmospheric composition of HD 209458b and HD 189733b, the two hot Jupiters most observed today. For these two planets, we have studied the sensitivity of the results to different subnetworks describing the reaction chemistry of nitrogen used in the field of combustion. We have shown that it is important to use a chemical network experimentally validated and not optimized for too specific conditions of composition, temperature and pressure. We have also developed a network up to C6 to study the complexification processes of organic matter for carbon-rich elemental abundances. Indeed, the study of an atmosphere with a C/O ratio > 1 was not possible so far without such a high temperature chemical network. Thanks to several collaborations, we gathered, for the first time in Europe, the skills to perform all the steps of a 1D atmospheric modeling: synthesis of UV stellar spectrum, determination of the thermal structure, study of the photochemistry and realization of synthetic spectra. We have also begun to measure the absorption cross sections at high temperature of important molecules for the atmosphere of hot exoplanets. These data did not exist before and are essential to model correctly the photodissociation processes that take place in their atmosphere. We started by measuring the carbon dioxide at temperatures ranging from 300 to 800 K over the range of wavelengths [115-220] nm. We observed that the absorption cross section of CO2 increases with the temperature at long wavelengths. We have shown that these new measures had an impact on the atmospheric composition of hot exoplanets, such as hot Neptunes. These measures are the first of a long-term work since absorption cross sections at high temperature of other molecules (H2O, NH3, HCN,...) must be determined. Finally, we have studied the influence of the horizontal atmospheric circulation on the composition of HD 209458b with a simple dynamic model and our robust chemical scheme. While the longitudinal thermal profile shows a temperature gradient up to 800 K, we have shown that the horizontal flow homogenizes differences between East and West limbs against chemical equilibrium. This result suggests that the transit spectra are interpretable through 1D models, which has been assumed since the first spectroscopic observations of transits but had never been demonstrated. This is reassuring for the potentialities of the telescopes EChO and JWST.nrpages: 250status: publishe