Atmospheric Erosion Caused by Stellar Coronal Plasma Flows on Terrestrial Exoplanets within Close-In Habitable Zones of Low Mass Stars

摘要

Since low mass M stars show a higher level of stellar activity compared to solar-like stars, and because of the closer orbital distance of their habitable zones compared to that of the Solar System, terrestrial exoplanets within M star habitable zones are expected to be much more strongly influenced by stellar winds and dense plasma ejected from the host star by coronal mass ejections. The efficiency of atmospheric erosion of CO_2-rich exoplanets,having the size and mass similar to that of the Earth, due to dense stellar plasma flows within close-in habitable zones of active M-type dwarf stars is investigated. Since M stars are active at the X-ray and EUV radiation (XUV) wavelengths over long time periods, we have applied a thermal balance model at various XUV flux input values for simulating the thermospheric heating by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by the CO_2 IR radiation in the 15μm band. Our study shows that intense XUV radiation of active M-stars, together with the photochemical production of excited atomic oxygen results in atmospheric expansion and extended exospheres which can interact with the stellar plasma flow. Using the calculated thermospheric neutral and ion densities, we applied a 3-D magnetohydrodynamic and a test particle model for calculating the non-thermal loss rates from the extended exospheres of magnetized and non-magnetized Earthlike exoplanets. The consequences of our preliminary results for the evolution of habitable planets within active M star environments are discussed.