THE INTERACTION OF THE STELLAR HIGH ENERGY RADIATION WITH THE CIRCUMSTELLAR MEDIUM

The purpose of this Thesis is to study how stellar high energy radiation affects exo- planetary atmospheres. High level of stellar high energy, X-rays and (vacuum end extreme) ultraviolet radiation may prevent planets’ habitability and limit the width of the habitability zone. Such radiation may ionize a star surrounding, affecting the chemistry and thermal bal- ance. The effects of X-rays can be wide ranging or relatively local. Soft X-rays are absorbed closer to the source than X-rays with energy greater than 1 keV. The absorption cross-section of hard X-rays are generally very small, so that such radiation may affect a whole galaxy to some extent. The product of photo-ionization is a cascade of fast photoelectrons, that in turn ionize and heat the medium through Coulomb collisions. Fast electrons may also excite the gas by means of non-thermal collisions, and thus populate high-lying rotational and vibrational levels of molecules such as e.g. H 2 , enhancing their low frequency emission.