2025A&A...696A.211S

Planet KELT-9 b has motivated a number of transit observations that have yielded a wealth of information, including absorption in several lines of hydrogen, lines of a number of heavy elements, and the first detection in the atmosphere of an exoplanet of absorption in the O I 777.4 nm line. At the same time, it has generated pioneering works on the aeronomy of the upper atmospheres of hot exoplanets, which opened new dimensions into existing models. The unique conditions for the interaction of the radiation of an A-class star with the atmosphere necessitate kinetic modeling of the excited levels of elements, primarily the hydrogen atom. In this work, we performed a 3D simulation of the upper atmosphere of KELT-9 b to include the effects of a close location of the Roche lobe, and the kinetics of the excited levels of hydrogen and other elements to include in the model additional heating by low energy photons of stellar flux. All these allowed us to reproduce the measured absorption in Hα, Hβ, Paβ, and O I lines in a self-consistent way. The main finding is that there is an intrinsic correlation between the heating of the atmosphere by far ultraviolet flux instead of extreme ultraviolet and X-ray, the observed absorption by excited levels of H I, and the overall mass loss from KELT-9 b.