Detection in Hα of a Supershell Associated with W4

From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10 degrees diameter field centered on this region in the Her line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. 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 OCl 352 (less than or similar to 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.