Introduction

Strong winds from massive stars can sweep up the ambient gas forming stellar wind bubbles, also called ring nebulae. Classically, ring nebulae around Wolf-Rayet (WR) stars have been modeled assuming a homogeneous interstellar medium (ISM), following Weaver et al. (1977). However, theory and observations have progressed to the point that, this simplification can no longer be justified. The evolution of massive stars has been studied by Maeder (1990). He shows tracks for 15–120 M⊙ in his plots (Figures 1-4). Main sequence (MS) stars between 25–40 M⊙ evolve to WR stars after passing through a red supergiant (RSG) phase. Observations of MS stars (Herrero et al. 1992) and WR stars (Willis 1991) reveal fast winds, as opposed to RSG stars (Humphreys 1991), where the winds are dense and slow (see also Chevalier & Liang 1989, Stencel et al. 1989). The above studies, suggest to us that the ISM initially encountered by a WR wind is far from homogeneous. This is the base of our three-wind model. In order to explain WR ring nebulae, we must take into account the history of the central stars, not just their interstellar environment. We have already presented a brief description of an analytic calculation of the dynamical behavior of the swept-up shell of RSG wind (Garcia-Segura. and Mac Low 1993). In this paper, we present numerical computations of the shell that follow it after instability sets in and it can no longer be modeled analytically.