La thermalisation des électrons dans une atmosphère stellaire

Abstract : In this thesis we draw a theoretical study of a stellar atmosphere, modelled as a parallel-plane slab irradiated on one face, where free electrons are not thermalised. Free electrons are characterized by their velocity distribution function : the electron distribution function (edf), which is calculated in addition to other atmospheric thermodynamical quantities. Our main objective is to understand the mechanism leading to the thermalisation of electrons, where the edf tends toward the Maxwell-Boltzmann distribution as long as elastic collisions dominate inelastic interactions of electrons with the plasma. This hypothesis is always accepted, in stellar atmospheres theory. Inelastic (collisional or radiative) processes disturb this equilibrium, and the edf can considerably deviate from the maxwellian equilibrium at high energies. Such deviations strongly modify atomic populations and the radiative field. Numerical computations consist in the comparison between three stellar atmosphere models: in local thermodynamic equilibrium (LTE), non-LTE models with thermalised electrons, and we relax the hypothesis of a priori thermalised electrons. This problem was solved in this thesis by using an hydrogen plasma and the main types of interactions found in stellar atmospheres. The kinetic equation of electrons was solved using a BGK model for elastic collisions, this model being extensively detailled in the thesis. Our main contribution consists in correctly solving the radiative transfer equation. We used, and especially developed, the code from the Transfert team at CRAL Observatoire de Lyon. We show that the edf considerably deviate from a maxwellian distribution over all non-LTE part in the stellar atmosphere. Finally, some extensions and astrophysical applications of this work are discussed.
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Astrophysique [astro-ph]. Université Claude Bernard - Lyon I, 2000. Français
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Loïc Chevallier. La thermalisation des électrons dans une atmosphère stellaire. Astrophysique [astro-ph]. Université Claude Bernard - Lyon I, 2000. Français. 〈tel-00006301〉

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