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.