Retrieval of molecular abundances and temperature-pressure profiles with high-resolution transmission spectroscopy in the near-infrared.

High-resolution (HR) transmission spectroscopy has been proven to be very effective in the detection of multiple molecular species in the atmospheres of extrasolar planets (Giacobbe et al., 2021). At HR, absorption bands of molecules are resolved into thousands of individual lines, so that species can be unambiguously identified by line matching (e.g. via cross-correlation) with planetary model templates, even though most of the single spectral lines are embedded in the photon noise.

The next challenge is to link the detection of molecular species to the chemical and physical properties of the exoplanet atmosphere. To this end, it is necessary to shift from the standard cross-correlation framework to a Bayesian log-likelihood Markov chain Monte Carlo framework, so as to infer fundamental properties such as the abundances of molecular species and the atmospheric pressure/temperature profile (Line et al., 2021). We present both the methodology and the first results on retrievals from transmission spectroscopy observations of warm/hot Jupiters carried out with the GIANO-B (R ~ 50000) near-infrared spectrograph by the GAPS (Global Architecture of Planetary System) consortium. From the derived volume mixing ratios of several molecules, we are able to derive the atmospheric C/H, O/H and C/O abundances, which are thought to be tracers of planet formation and migration scenarios.