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Wolf-Rayet stars are in the final stages of stellar evolution. In order to explain the ring nebulae observed around them, we must take into account their history, as well as their surroundings. Doing this, we develop a three-wind model. The first wind comes from the main-sequence star. It is a fast wind (2000 km/s) that sweeps up the interstellar medium. When the main-sequence star evolves into a red supergiant (RSG), its wind becomes slow (10 km/s) and dense, carrying much mass but little momentum. Finally, the fast wind from the Wolf-Rayet star sweeps up the RSG wind, producing the observed bubble.
Wolf-Rayet ring nebulae are often observed to be clumpy or filamentary. Examples include NGC 6888 and RCW 58. The three-wind model naturally predicts fragmentation of the shell of swept-up RSG wind at the boundary between the RSG wind and the main-sequence bubble. The sudden drop in density at the boundary allows the shell to accelerate as it crosses the boundary. The strong acceleration in turn produces Rayleigh-Taylor instabilities in the shell, fragmenting it into isolated clumps. The hot shocked wind flows freely out between the clumps, driving the outer shock at high velocity.
Here, we present hydrodynamical simulations with the code ZEUS-3D to follow the growth of Rayleigh-Taylor instabilities in the swept up shell. We find that our models beautifully reproduce the observed morphology of nebulae such as NGC 6888 and RCW 58. Most of the H$\alpha$ emission comes from the fragmented shell of swept up RSG wind, while the filamentary [O~III] emission comes from the outer shock bursting through the shell.
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