  Observations of ultra-hot Jupiters indicate the existence of thermal
inversion in their atmospheres with day-side 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 on the thermal structure of their upper atmospheres.We
aim at exploring 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.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 day-side atmosphere of the highly irradiated ultra-hot
Jupiter WASP-19b with Teq ~ 2700 K. At these high temperatures we find that H2O
dissociates thermally at pressure below 10^-2 bar. The inverted
temperature-pressure profiles of WASP-19b show the evidence of CO emission
features at 4.5 micron in its secondary eclipse spectra.We find that the
atmosphere ofWASP-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 tau = 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 makes H2O
more difficult to identify in WASP-19b.
