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Session 61 - New Views of the Magellanic Clouds.
Display session, Wednesday, June 12
Great Hall,
During its evolution, a massive star loses mass via stellar winds. A fast stellar wind may sweep up the ambient medium into a shell, appearing as a ``ring nebula" around the central star. While ring nebulae around Wolf-Rayet stars have received considerable attention in recent years, ring nebulae around O and B stars are far less well-explored. This is because very few well-defined rings around OB stars are known; in our Galaxy only two cases are known, the Bubble Nebula and NGC6164-5.
Last year we discovered two ring nebulae around blue supergiants, Sk-69 279 (O9f; V=12.8 mag) and Sk-69 271 (B2; V=12.0 mag), in the Large Magellanic Cloud (Weis et al. 1995, RevMexAASC 3, 237). Both nebulae have diameter \sim19'', corresponding to \sim5 pc. To investigate the origin of these nebulae, we obtained long-slit H\alpha+[N II] echelle observations with the 4m telescope at the Cerro Tololo Inter-American Observatory. The nebula around Sk-69 279 shows line-split indicating an expansion velocity of 27\,km s^-1, while the nebula around Sk-69 271 shows no line-split, suggesting an expansion velocity <15\,km s^-1. Assuming that t = 0.5 r/v, the dynamic age of these nebulae are 1\times10^5 yr and >2\times10^5 yr, respectively.
The most tale-telling information comes from the [N II]/H\alpha ratio. Both nebulae show [N II]/H\alpha ratios significantly higher than those of the background H II emission. This behavior is typical for ring nebulae around WR stars or luminous blue variables that contain stellar nucleosynthesis processed material. Therefore, we conclude that the ring nebulae around Sk-69 279 and Sk-69 271 must be ``circumstellar bubbles" containing processed stellar material. These two blue supergiants must have evolved past the red supergiant phase. The chemical composition of these two ring nebulae could place constraints on models of stellar evolution.
