Bridging the gap between theory and observation: interpreting the transmission signatures of the ultra-hot Jupiter WASP-121b with global circulation models and 3D radiative transfer

Joost Wardenier ( Université de Montréal )

Ultra-hot Jupiters (UHJs) are the best targets for in-depth characterization at high spectral resolution. However, their atmospheres are vastly non-uniform in terms of chemistry, temperature, dynamics, and cloud cover, which makes their observed spectra challenging to interpret. Resolving their transmission spectra as a function of orbital phase provides crucial insights into the “3D nature” of these planets. In this contribution, we present the first ever phase-resolved water and CO absorption signals of an UHJ, based on three transits of WASP-121b observed with GEMINI-S/IGRINS. While CO absorption remains equally strong during the transit, the water signal displays significant variation, suggesting water dissociation or cloud formation on one of the limbs of the planet. Also, both molecules display different Doppler shifts, with CO becoming more blueshifted and water becoming more redshifted during the transit. We demonstrate that this behaviour is the result of the unique spatial distribution of water and CO, in combination with strong atmospheric drag. These findings are backed up by the Fe signature of WASP-121b that was previously observed with VLT/ESPRESSO. We compare our observations to a suite of global circulation models and their corresponding 3D spectra, to paint an accurate, multi-dimensional picture of the limb region of WASP-121b.