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, which contains indications about the planet formation history. With a wavelength coverage going from 0.95 to 2.5 microns, the high-resolution (R sim 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 programme. 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$_2$O and CO at a Doppler shift of around -6 km.s$^ $, which is consistent with previous observations of the planet. Finding that a deep cloud deck is favoured, we measured a mass mixing ratio (MMR) log(H$_2$O)$_ MMR $ = -4.52 pm 0.77 and log(CO)$_ MMR $ = -3.09 pm 1.05 consistent with a sub-solar metallicity to more than 1sigma . We report 3sigma upper limits for the abundances of C$_2$H$_2$, HCN, and OH. We estimated a C/O ratio of 0.94 pm 0.39 (sim 1.7 pm 0.7 times solar, with errors indicated corresponding to the 2sigma 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$_2$O and CO, as well as our upper limits for C$_2$H$_2$, HCN and OH, we found that our results were consistent with a C/O ratio between 1 and 2 times solar, and hence with our C/O estimation. Finally, we found indications of asymmetry for both H$_2$O and CO when investigating the dynamics of their signatures, pointing to a complex scenario possibly involving both a temperature difference between limbs and the presence of clouds being behind the asymmetry that this planet is best known for.