First 3D Radiation-Hydrodynamic Simulations of Wolf-Rayet Winds

Abstract

Classical Wolf Rayet (WR) stars are direct supernova progenitors undergoing vigorous mass-loss. Understanding the dense and fast outflows of such WR stars is thus crucial for understanding advanced stages of stellar evolution, the dynamical feedback of massive stars on their environments, and characterizing the distribution of black hole masses. In this paper, we develop first time-dependent, multi-dimensional, radiation-hydrodynamical models of the extended optically thick atmospheres and wind outflows of hydrogen-free classical WR stars. A flux limiting radiation hydrodynamics approach is used on a finite volume mesh to model WR outflows. The opacities are described using a combination of tabulated Rosseland mean opacities and the enhanced line opacities expected within a supersonic flow. For high-luminosity models, a radiation-driven, dense, supersonic wind is launched from deep sub-surface regions associated with peaks in the Rosseland mean opacity. For a model with lower luminosity, on the other hand, the Rosseland mean opacity is not sufficient to sustain a net-radial outflow in the sub-surface regions. Rather, what develops in this case is a "standard" line-driven wind launched from the optically thin regions above an extended and highly turbulent atmosphere. We thus find here a natural transition from optically thick outflows of classical WR stars to optically thin winds of hot, compact sub-dwarfs; in our simulations this transition occurs approximately at a luminosity that is about 40% of the Eddington luminosity. Because of the changing character of the wind-launching mechanism, this transition is also accompanied by a large drop (on the low-luminosity end) in average mass-loss rate.