Theoretical undestanding of atmospheric escape of close-in planets driven by EUV photoionization

The intense extreme ultraviolet radiation heats the upper atmosphere of close-in exoplanets and drives the atmospheric escape through photoionization of hydrogen atoms. The intense atmospheric escape is a key process to understand the evolution of close-in exoplanets. The mass loss rate depends on the planetary environment and parameters (e.g. UV flux at the planet, planetary radius, mass).The escape process can be characterized by the gravity, photoheating/cooling, and gas expansion. We introduce the relevant physical quantities which describe the dominant physics in the atmosphere. Particularly the introduction of a characteristic temperature describing how much the atmosphere is heated by photoionization can be used to understand physical quantities of atmospheric escape, including the mass-loss rate. We find that the equilibrium temperature for phootoionization of hydrogen and the characteristic temperature determine whether the system becomes energy-limited or recombination-limited. The atmospheric temperature can be given by these temperatures.We derive an analytic formulae for mass-loss rate and efficiency driven by EUV photoinization which can explain the results from 1D hydrodynamic simulations. We also classify close-in exoplanets with Ly-a, Ha, and Helium triplet observations which reflects the thermo-chemical structure of the upper atmosphere. Our classification suggests the future candidate to observe absorption by the upper atmosphere with longer photoionization timescale and larger mass-loss rates.