GSH 23.0−0.7+117: A Neutral Hydrogen Shell in the Inner Galaxy

GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in theVLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell wasdetected. The expansion velocity and systemic velocity were determined throughthe systematic behavior of the HI emission with velocity. The center of theshell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume ofthe half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the mediumin which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr,with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} ergcan be provided by the stellar wind of a single O4 to O8 star over the age ofthe shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density(S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuumluminosity generated inside the shell. This upper limit implies a maximum ofone O9 star (O8 to O9.5 taking into account the error in the distance) insidethe HI shell, unless most of the incident ionizing flux leaks through the HIshell. To allow this, the shell should be fragmented on scales smaller than thebeam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble hasburst (as suggested by the HI channel maps), agreement between the energy andionization requirements is even less likely. The limit set by the non-detectionin the continuum provides a significant challenge for the interpretation ofGSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicableto other Galactic HI shells that have not been detected in the continuum.Comment: 18 pages, 6 figures. Figures 1 and 4 separately in GIF format. Accepted for publication in Astrophysical Journa