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R.A. Gruendl, Y.-H. Chu (UIUC)
Wolf-Rayet (WR) stars are often found to be surrounded by ring nebulae composed of circumstellar and/or interstellar material. In these objects, comparison of emission from different lines often show different morphologies. In particular, in NGC 6888, the [O III] emission is more extended compared to H\alpha and [N II] emission. This difference in morphology has been explained in models of the formation and evolution of ring nebulae around massive stars by Garcia-Segura et al. (1996). They suggest that an interstellar bubble is formed around a massive star during its main sequence stage and a circumstellar bubble will be formed after the star has made a transition from a red supergiant to a WR star. They further predict that evolved circumstellar bubbles will fragment and form ``break-out'' regions where the shock front (as traced by [O III] emission) is leading the fragmented circumstellar shell material (traced by H\alpha and [N II] emission).
We have imaged [O III] and H\alpha emission from the 6 most well defined WR ring nebulae in the northern sky (S 308, NGC 2359, NGC 6888, G2.4+1.4, WR 128, and WR 134). Chemical abundances measured in these nebulae indicate that all except NGC 2359 are circumstellar bubbles. The four largest circumstellar bubbles (r>6 pc) show [O III] limb features exterior to the limb-brightened H\alpha shell. The linear offset between the [O III] and H\alpha fronts is ~0.02--0.15 pc. These results are qualitatively consistent with model predictions of Garcia-Segura et al. Therefore, we suggest that an observed separation between [O III] and H\alpha shell rims can be used to determine that a ring nebula around a massive star is an evolved circumstellar bubble.
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