Fig. 3: Atmospheric constraints from the chemical-equilibrium and free-chemistry retrievals.
From: A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b
a, Retrieved temperature–pressure profiles with 1σ and 2σ contours for the chemical equilibrium with free temperature–pressure profile (blue), radiative–convective–thermochemical equilibrium (1D-RCTE, red) and free chemistry with thermal dissociation (green) retrievals. The retrieved temperature–pressure profiles are consistent between the retrievals and show an inversion in the pressure range that is constrained from the observations, as shown by the contribution functions at 0.85 (dot-dashed grey line), 1.82 (dashed brown line) and 2.83 μm (orange line). The temperature–pressure profile of WASP-18b is above the CaTiO3 condensation curve50 (dashed black line) at almost all pressures, which motivates the presence of a temperature inversion caused by TiO, as Ti is available in gas form. The dayside average temperature–pressure profile of the τdrag = 103 s SPARC/MITgcm (dashed white line) is computed from the viewing angle average of T(P)4 and shown for comparison. b–d, We also show the posterior probability distributions of the atmospheric metallicity [M/H] (b), C/O ratio (c) and area fraction AHS (d). The area fraction AHS is a scaling factor applied to the thermal emission spectrum to compensate for the possible presence of a concentrated hotspot contributing to most of the observed emission51. All methods retrieve metallicities consistent with solar at 1σ. The retrieved C/O 3σ upper limits are of 0.6 and 0.2 for the chemical equilibrium with free temperature–pressure profile and the 1D-RCTE retrievals, respectively. Finally, we find that the area fraction AHS is consistent with 1 when allowing the temperature–pressure profile to vary freely, indicating the lack of a concentrated hotspot on the dayside contributing to most of the observed emission.
