Caractérisation des atmosphères exoplanétaires par spectroscopie haute résolution

Thousands of exoplanets have been confirmed in the last two decades, and yet observational constraints on their compositions have only been obtained for a few hundred of them so far. Current detection methods only give access to the radius and mass of a planet and therefore to its bulk density, which generally induces large degeneracies in terms of composition and structure. Characterization of exoplanet atmospheres has therefore emerged as a challenge for the exoplanet scientific community. Constraining the composition, dynamics, and overall structure of an exoplanet atmosphere allows us to infer its formation and evolution history and to put our Solar system in a broader context through comparative planetary science. Transmission spectroscopy has emerged as a powerful method to characterize exoplanetary atmospheres: when an exoplanet transits its host star, part of the stellar flux passes through the exoplanet’s atmosphere, imprinting the signature of its molecular components on the observed flux. Observing these transmission spectra with ground-based high-resolution spectrometers allows us to detect the species present in the atmosphere and probe the dynamics of the atmosphere by resolving the individual lines, all while being less sensitive to clouds and hazes opacities than space-based observations at medium-resolution. However, the analysis of ground-based observations requires state-of-the-art data reduction and processing methods to correct for the Earth atmosphere and the background host star contributions and to extract the faint planetary signal whose individual lines amplitude are orders of magnitude weaker than the noise. During this thesis, I worked on the atmospheric characterization of fifteen exoplanets observed with the SPIRou instrument, a near-infrared spectropolarimeter at the Canada France Hawaii Telescope. I developed a complete data processing and analysis pipeline to search for atmospheric escape and molecular signatures in the studied targets, taking into account subtle effects such as the deformation of the stellar lines induced by the exoplanet transit. I searched for the metastable He triplet signature at 1083.3 nm (in vacuum), a near-infrared probe for atmospheric escape, and constrained the escaping atmosphere properties using a combination of 1D hydrodynamic escape and Non Local Thermodynamic Equilibrium radiative transfer models. Using Cross Correlation Function and Nested Sampling methods coupled with a 1D radiative-convective equilibrium code and a high-resolution radiative transfer model, I searched for the presence of H2O, CO, and CH4, and constrained their abundances in the targeted planets. Applying the same reduction pipeline on a set of targets observed with the same instrument further allowed me to provide homogeneously retrieved constraints on these targets, paving the way toward a statistical understanding of exoplanets in terms of atmospheric composition and structure.

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