Extended Data Fig. 5: Abundance constraints from the free-chemistry retrievals.
From: A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b
Probability posteriors of the deep abundance of various species considered for the free chemistry and temperature with (blue, HyDRA) and without (red, POSEIDON) thermal dissociation. We also show the median retrieved VMR profiles from the chemical equilibrium with free temperature–pressure profile retrieval (black line, SCARLET). The pressure range investigated by the observations (about 0.01–1 bar; see Fig. 3) is indicated by the dashed grey lines. The only species independently detected is H2O, which is found to be consistent with the retrieved chemical-equilibrium abundance when considering the effect of thermal dissociation. All other species considered are found to be unconstrained, although consistent with chemical-equilibrium predictions. The photosphere as predicted by our radiative–convective model is around 50 millibar, but the retrievals infer the deep molecular abundances.
