Atmospherix

III. Estimating the C/O ratio and molecular dynamics at the limbs of WASP-76 b with SPIRou^★

¹ IRAP, UMR5277 CNRS, Université de Toulouse, UPS, Toulouse, France
e-mail: thea.hood@irap.omp.eu
² Department of Physics, University of Oxford, Oxford OX1 3RH, UK
³ Université Paris-Saclay, UVSQ, CNRS, CEA, Maison de la Simulation, 91191 Gif-sur-Yvette, France
⁴ Université Côte d’Azur, Observatoire de la Côte d'Azur, CNRS, Lagrange, CS 34229, Nice, France
⁵ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Université de Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France
⁷ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, 5 place Jules Janssen, 92195 Meudon, France
⁸ Institut d’Astrophysique de Paris, CNRS, UMR 7095, Sorbonne Université, 98 bis bd Arago, 75014 Paris, France
⁹ Laboratório Nacional de Astrofísica, Rua Estados Unidos 154, 37504-364, Itajubá, MG, Brazil
¹⁰ Laboratoire de Météorologie Dynamique/IPSL, CNRS, Sorbonne Université, Ecole Normale Supérieure, Université PSL, Ecole Polytechnique, Institut Polytechnique de Paris, 75005 Paris, France
¹¹ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France

Received: 7 December 2023
Accepted: 22 March 2024

Abstract

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 µm, 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 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 H₂O and CO at a Doppler shift of around −6 km s⁻¹, 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(H₂O)_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 C₂H₂, HCN, and OH. We estimated a C/O ratio of 0.94 ± 0.39 (~ 1.7 ± 0.7 × 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 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 × 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 possibly involving both a temperature difference between limbs and the presence of clouds being behind the asymmetry that this planet is best known for.