Non-local thermodynamic equilibrium effects determine the upper atmospheric temperature structure of the ultra-hot Jupiter KELT-9b

Several results indicate that the atmospheric temperature of ultra-hot Jupiter KELT-9b in main line formation region is a few thousand degrees higher than predicted by self-consistent models. We test whether non-local thermodynamic equilibrium (NLTE) effects are responsible for presumably temperature. employ Cloudy NLTE radiative transfer code to self-consistently compute upper temperature-pressure (TP) profile KELT-9b, assuming solar metallicity. The TP $\approx$2000 K hotter obtained with previous models local (LTE). In particular, 1-10$^{-7}$ bar range increases from $\approx$4000 $\approx$8500 K, remaining roughly constant at lower pressures. find high atmosphere driven principally modifying Fe and Mg level populations, which strongly influence thermal balance. LTE synthetic transmission spectra on basis profiles computed NLTE, respectively, finding model generally produces stronger absorption lines (up 30%), largest ultraviolet. compare spectrum observed H$\alpha$ H$\beta$ obtaining an excellent match, thus supporting our results. can be used guide future observations aiming detecting features planet's spectrum. Metals, such as Fe, shape structure affect mass-loss rates derived it. Finally, call checking this case also cooler planets.