ATMOSPHERIX: III- Estimating the C/O ratio and molecular dynamics at the limbs of WASP-76 b with SPIRou

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 (R∼70 000) 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 H_2O 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(H_2O)_MMR = -4.52 ± 0.77 and log(CO)_MMR = -3.09 ± 1.05 consistent with a sub-solar metallicity to more than 1σ. We report 3σ upper limits for the abundances of C_2H_2, 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 2σ 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 H_2O and CO, as well as our upper limits for C_2H_2, 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 H_2O 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.