A morphological diagnostic for dynamical evolution of Wolf-Rayet bubbles

We have observed Hα and [O III] emission from eight of the most well-defined Wolf-Rayet ring nebulae in the Galaxy. We find that in many cases the outermost edge of the [O III] emission leads the Hα emission. We suggest that these offsets, when present, are due to the shock from the Wolf-Rayet bubble expanding into the circumstellar envelope. Thus, the details of the WR bubble morphology at Hα and [O III] can then be used to understand better the physical condition and evolutionary stage of the nebulae around Wolf-Rayet stars, as well as to place constraints on the nature of the stellar progenitor and its mass-loss history.