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S. Basu, D. Johnstone, P.G. Martin (CITA)
Recent observations have revealed a complete, well-resolved superbubble in our Galaxy. Lying above the W4 \ion{H}{2}~ region is a large cavity in \ion{H}{1}, observed by Normandeau, Taylor, and Dewdney (1996) as part of the Canadian Galactic Plane Survey. An outer shell is also observed in H\alpha by Dennison, Topasna, and Simonetti (1997). The superbubble is apparently powered by nine O stars in the stellar cluster OCl 352, which sits at the base of the conical cavity. We model the evolution and the current shape of the W4 superbubble using the semi-analytic Kompaneets (1960) approximation, and calculate the ionization structure of the elongated bubble and external medium. The extreme elongation of the bubble over the scales it has expanded imply a relatively low scale height of local gas: H ~q 25 pc. We discuss the various implications if interstellar gas in a star-forming region has such a low scale height in comparison to the global average for the ISM (H \gtrsim 100 pc near the Galactic plane). For example, Lyman continuum photons from an OB association are much more likely to penetrate to large heights above the Galactic plane. Additionally, we demonstrate that the sweeping up of ambient matter into a shell by stellar winds or supernovae initially hinders the escape of Lyman continuum photons, but eventually aids their escape. In the case of the nine O stars associated with W4, we estimate that approximately 15% of Lyman continuum photons currently escape the bubble and swept-up shell. We discuss the implication for maintenance of the Warm Ionized Medium, or Reynolds layer, in our Galaxy.
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