A hydrodynamic study of the escape of metal species and excited hydrogen in the atmosphere of the hot Jupiter WASP-121b

The heating by photoionization in the thermosphere of short-period exoplanets can drive hydrodynamic escape, which is key to understanding the evolution of planetary atmospheres and explaining transit observations. Besides powering atmospheric escape, the energy deposited by extreme UV photons from the host star can also be radiated away through collisionally excited atomic spectral lines. In addition, metals and excited state hydrogen, which have lower ionization potentials than 13.6 eV, can absorb the longer wavelength photons in the stellar spectrum. These two factors, in addition to Roche lobe overflow, can cause the mass loss rate to exceed the traditional energy-limited value. In the near UV and optical transmission spectrum of the hot Jupiter WASP-121b, recent observations have detected strong absorption features of Mg, Fe, Ca, and Hα, extending outside the planet’s Roche lobe. Studying these atomic signatures can directly trace the escaping atmosphere and constrain the thermal processes in the upper atmosphere. To understand these features, we construct a sophisticated forward model by expanding the capability of the exoplanet hydrodynamic atmosphere code introduced in Koskinen et al. 2013, to include important processes of atomic metal species and excited hydrogen. Using this model, we can reproduce and interpret detected atomic features in the transmission spectrum of WASP-121b self-consistently.