Dynamical Evolution and Ionization Structure of an Expanding Superbubble: Application to W4

Recent observations have revealed a superbubble associated with the youngstellar cluster OCl 352 near the W4 H II region: a void in H I emission(Normandeau, Taylor, and Dewdney), and a bright shell in H alpha emission(Dennison, Topasna, and Simonetti). We investigate the hypothesis that thebubble is blown by stellar winds from the O-type stars in the association. TheKompaneets approximation is adapted to model a wind-blown bubble in astratified interstellar medium. We describe some general principles necessaryfor understanding the dynamics of an expanding bubble and the associatedionization structure in a stratified atmosphere. The Kompaneets model can beused to determine the mean scale height of the ambient medium as well as theage of the bubble. The ionization structure also places constraints on theambient density near the cluster. We also estimate the surface brightness ofthe shell and the fraction of ionizing photons which escape the bubble. Theprescription we use can be applied to any observed bubble that is blown by theeffectively continuous energy output of stellar winds or multiple supernovae.Application to the W4 superbubble shows that the mean scale height of theambient gas around the cluster is remarkably small, 25 pc for a clusterdistance of 2.35 kpc. The age of the bubble is estimated to be about 2.5 Myr,consistent with the notion that the bubble is blown by stellar winds from avery young cluster in which no supernovae have yet occurred.Comment: 36 pages, 12 figures, AASTeX, to appear in the Astrophysical Journa