ESCAPING PARTICLE FLUXES IN THE ATMOSPHERES OF CLOSE-IN EXOPLANETS. I. MODEL OF HYDROGEN

Abstract

A multi-fluid model for an atomic hydrogen-proton mixture in the upper\natmosphere of extrosolar planet is presented when the continuity and momentum\nequations of each component have been already solved with an energy equation.\nThe particle number density, the temperature distribution and the structure of\nvelocity can be found by means of the model. We chose two special objects, HD\n209458b and HD 189733b, as discussion samples and the conclusion is that their\nmass loss rates predicted by the model are in accordance with those of\nobservation. The most important physical process in coupling each component is\ncharge exchange which tightly couples atomic hydrogen with protons. Most of the\nhydrogen escaping from hot Jupiters is protons, especially in young star-planet\nsystem. We found that the single-fluid model can describe the escape of\nparticles when the mass loss rate is higher than a few times $10^{9}$ g/s while\nbelow $10^{9}$ g/s the multi-fluid model is more suitable for it due to the\ndecoupling of particles. We found that the predicted mass loss rates of HD\n189733b with the assumption of energy-limit are a factor of 10 larger than that\ncalculated by our models due to the high ionization degree. For the ionized\nwind which is almost compose of protons, the assumption of energy-limit is no\nlonger effective. We fitted the mass loss rates of the ionized wind as a\nfunction of $F_{UV}$ by calculating the variation of the mass loss rates with\nUV fluxes.\n