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Measuring the abundances of C- and O-bearing species in exoplanet atmospheres enables us to constrain the C/O ratio, that contains indications about the planet formation history. With a wavelength coverage going from 0. 95 to 2. 5 microns, the high-resolution (R70 000) spectropolarimeter SPIRou can detect spectral lines of major bearers of C and O in exoplanets. Here we present our study of SPIRou transmission spectra of WASP-76 b acquired for the ATMOSPHERIX program. We applied the publicly available data analysis pipeline developed within the ATMOSPHERIX consortium, analysing the data using 1-D models created with the petitRADTRANS code, with and without a grey cloud deck. We report the detection of H₂O and CO at a Doppler shift of around -6 km. s^-1, consistent with previous observations of the planet. Finding a deep cloud deck to be favoured, we measured in mass mixing ratio (MMR) log (H₂O) ₌₌ₑ = -4. 52 0. 77 and log (CO) ₌₌ₑ = -3. 09 1. 05 consistent with a sub-solar metallicity to more than 1. We report 3 upper limits for the abundances of C₂H₂, HCN and OH. We estimated a C/O ratio of 0. 94 0. 39 (1. 7 0. 7 x solar, with errors indicated corresponding to the 2 values) for the limbs of WASP-76 b at the pressures probed by SPIRou. We used 1-D ATMO forward models to verify the validity of our estimation. Comparing them to our abundance estimations of H₂O and CO, as well as our upper limits for C₂H₂, HCN and OH, we found that our results were consistent with a C/O ratio between 1 and 2 x solar, and hence with our C/O estimation. Finally, we found indications of asymmetry for both H₂O and CO when investigating the dynamics of their signatures, pointing to a complex scenario involving possibly both a temperature difference between limbs and clouds being behind the asymmetry this planet is best known for.
Hood et al. (Thu,) studied this question.