Context. Observations of ultra-hot Jupiters indicate the existence of thermal inversion in their atmospheres, with dayside temperatures greater than 2200 K. Various physical mechanisms such as non-local thermal equilibrium, cloud formation, disequilibrium chemistry, ionisation, hydrodynamic waves, and associated energy have been omitted in previous spectral retrievals, while they play an important role in the thermal structure of their upper atmospheres.
Aims: We aim to explore the atmospheric properties of WASP-19b to understand its largely featureless thermal spectra using a state-of-the-art atmosphere code that includes a detailed treatment of the most important physical and chemical processes at play in such atmospheres.
Methods: We used the one-dimensional line-by-line radiative transfer code PHOENIX in its spherical symmetry configuration including the BT-Settl cloud model and C/O disequilibrium chemistry to analyse the observed thermal spectrum of WASP-19b.
Results: We find evidence for a thermal inversion in the dayside atmosphere of the highly irradiated ultra-hot Jupiter WASP-19b, with T_eq ~ 2700 K. At these high temperatures we find that H₂O dissociates thermally at pressures below 10^-2 bar. The inverted temperature-pressure profiles of WASP-19b show evidence of CO emission features at 4.5 μm in its secondary eclipse spectra.
Conclusions: We find that the atmosphere of WASP-19b is thermally inverted. We infer that the thermal inversion is due to the strong impinging radiation. We show that H₂O is partially dissociated in the upper atmosphere above about τ = 10^-2, but is still a significant contributor to the infrared opacity, dominated by CO. The high-temperature and low-density conditions cause H₂O to have a flatter opacity profile than in non-irradiated brown dwarfs. Altogether these factors make H₂O more difficult to identify in WASP-19b. We suggest that the state-of-the-art PHOENIX model atmosphere code is well suited to the study of this new class of extrasolar planets, ultra-hot Jupiters.