Time-dependent models of radiatively driven stellar winds. I - Nonlinear evolution of instabilities for a pure absorption model

The authors describe results of numerical radiation-hydrodynamics simulations of the nonlinear evolution of instabilities in radiatively driven stellar winds. The wind is idealized as a spherically symmetric, isothermal flow driven by pure absorption of stellar radiation in a fixed ensemble of spectral lines. The simulations indicate that there is a strong tendency for the unstable flow to form rather sharp rarefactions in which the highest speed material has very low density. The growth of wave perturbations thus remains nearly exponential well beyond the linear regime, until the waves are kinematically steepened into strong shocks. The strongest shocks here are reverse shocks that arise to decelerate high-speed, rarefied flow as it impacts slower material that has been compressed into dense shells. The subsequent wind evolution shows a slow decay of the shocks and the gradual thermal decompression and interaction of the dense shells.