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Radiative shocks in stellar atmosphere: Structure and turbulence amplification

Abstract
The structure of radiative shock waves propagating through partially ionized hydrogen gas in stellar atmospheres is discussed. Basic equations including the radiation transfer and the method of their self-consistent solution are described. The most striking result is that the ratio of the radiative flux to the total energy flux of the shock wave very rapidly enlarges with increasing upstream velocity, so that for Mach number larger than 7, the major part of the shock energy is irreversibly lost due to dissipation processes. The understanding of the "missing temperature", called microturbulence by the astrophysicists, which appears when we want to modelling the width of stellar line profiles, is discussed. It is shown that the turbulence amplification in the atmosphere of a radially pulsating star is not only due to the global compression of the atmosphere during the pulsation. Strong shock waves propagating from the deep atmosphere to the very low density layers also play a role in the turbulence variation, especially when they become very strong i.e., hypersonic. For shocks, the predicted turbulence amplification predicted by classical models is overestimated with respect to stellar observations when the compression rate becomes larger than 2 which corresponds to a limit Mach number near 2. Thus, when radiative effects take place, the present turbulence amplification theory breaks down. A new approach is required.