On Radiative Dissipation in Stellar Turbulent Flows

The purpose of this paper is to study the exchange of momentum between photons and material particles in a stellar turbulent flow located far away from the outer boundary of the star. The flow is assumed to possess a wide interval of characteristic scales, such that the largest turbulent eddies are optically thick while the smallest ones are optically thin. An asymptotic analysis of the radiation hydrodynamics transport equations is performed in the limit of small velocities. All of the angular information carried by the radiative intensity is kept, and an exact expression for its perturbed state is derived. From it, an expression for the radiative force exerted by photons on matter is deduced. This force reverts to the standard radiative viscous stress in the optically thick regime and takes the form of a return-to-the-mean term in the opposite regime. The transition between the two is controlled by a local average of the velocity field taken on a domain of the size of a photon mean free path. Besides, the radiative force is found to always dissipate kinetic energy. But, as opposed to a purely viscous situation, the corresponding dissipation has an upper bound that is reached in the optically thin regime. All of these results are valid not only in the gray case but also for frequency-dependent opacities, including when true and scattering components are present. To validate these results, Monte Carlo simulations are performed on a simplified configuration.