Presentation #404.01 in the session Catching Big Air: Giant Exoplanet Atmospheres.
The Ultra-Hot Jupiter WASP-121b is one of the three planets with the highest potential for mass loss by atmospheric escape among the known transiting giant planets for which the system parameters are well constrained. 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 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 show that WASP-121b resides in an intermediate escape regime between stellar XUV-driven photoevaporation and rapid Roche lobe overflow, and demonstrate how the observations can be used to constrain the ionization state and energy balance of its atmosphere. Finally, we contrast the NUV observations of WASP-121b with NUV transmission spectra of more moderate Hot Jupiters HD209458b and HD189733b.