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 wantto modelling the width of stellar line profiles, is discussed.It is shown that the turbulenceamplification in the atmosphere of a radially pulsating star is not onlydue to the global compression of the atmosphere during thepulsation. Strong shock waves propagating from the deep atmosphereto the very low density layers also play a role in the turbulencevariation, especially when they become very strong i.e., hypersonic.For shocks, the predicted turbulence amplification predicted by classicalmodels is overestimated with respect to stellar observationswhen the compression rate becomes larger than 2 which corresponds to alimit Mach number near 2. Thus, when radiative effects take place, thepresent turbulence amplification theory breaks down. A new approach isrequired.