Abstract from paper published in Astrophysical Journal Letters, 458, L1 (1996).

Detection in H-alpha of a Supershell Associated with W4

Brian Dennison, Gregory A. Topasna, and John H. Simonetti
Martin Observatory, Institute for Particle Physics and Astrophysics,
and Department of Physics,
Virginia Polytechnic Institute and State University,
Blacksburg, VA 24061

From HI observations, Normandeau, Taylor and Dewdney have identified a possible galactic chimney emanating from W4. We observed a 10 degree diameter field centered on this region in the H-alpha line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of HII, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk. The shell appears to close 6 degrees (or about 230 pc) above the star cluster, and at a galactic latitude of 7 degrees. The shell is quite elongated with its major axis approximately perpendicular to the galactic plane, as predicted for a superbubble formed in a stratified galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl352 (less than about 2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.