Modeling the near-UV signatures of escaping ultra-hot Jupiter atmospheres

The Ultra-Hot Jupiter WASP-121b is among the known transiting planets with the highest potential for atmospheric escape. This is confirmed by transmission spectra at different wavelengths that show evidence of the escape of hydrogen and metals from the planet’s atmosphere, with transit depths that exceed the Roche lobe in several absorption lines. In particular, the near-ultraviolet (NUV) transit depth of the planet, arising mostly from absorption by iron and magnesium ions, is higher relative to the Roche lobe than the NUV transit depths of more moderate Hot Jupiters. We simulate the atmosphere of WASP-121b by using a photochemical model for the lower and middle atmosphere and a multi-species model of hydrodynamic escape for the upper atmosphere. Our escape model has been updated to include many of the detected heavy elements as well as heating and cooling by key metal lines. It has also been coupled to Monte Carlo radiative transfer and detailed balance models to calculate the population of excited hydrogen atoms. We discuss the NUV and other observations in the context of the results to constrain the energy balance in the planet’s upper atmosphere. We then apply the model to the ultra-hot Jupiter WASP-189b and discuss the interpretation of the NUV transmission spectrum of this planet observed recently by the Colorado Ultraviolet Transit Experiment (CUTE) 6U cubesat.