noise added determines whether or not a source of the specified size and luminosity would be detected in our survey. Although rather conservative, this procedure does not reflect non-detection associated with confusion due to neighboring emission line sources. We suspect that the ambiguity related to objects bordering the WR star may often dominate the “error budget” for non-detection of WR shells. For this reason, the detection limit indicated in Figure 3 should be interpreted as a best-case scenario, appropriate when the WR star is relatively isolated but otherwise just a lower limit.

Figure 3 reveals that there is a gap between the detection limit and the H$\alpha$ emission from Group I shells. Why don’t we detect fainter shells? One possible explanation is that the larger shells are R-type, with many massive stars responsible for their ionization. Typically Galactic WR shells expand at 10-50 km s$^{\rm -1}$ (Marston 1995), therefore a shell would need $\geq$4 $\times$ 10$^{\rm 5}$ years to expand to a radius of 20 pc. If the WR phase lasts 10$^{\rm
4}$-10$^{\rm 5}$ years (Marston 1995) shells larger than 20 pc would no longer contain a WR star. The problem with this explanation is that these timescales hold true 