Brandt ci at. have shown that consistency in the combined observations of the Gum Nebula requires a giant H ii region, presumably formed by the Vela X supernova explosion. Mouison and Sartori had concluded on the basis of their He ii fluorescence theory of Type I supernovae that a giant H ii region would be formed as result of the ultraviolet burst. (Bottcher ci at., by integrating over the light curve, expect a smaller H ii region.) We present here in brief some consequences of the fluorescence model as illustrated by the Vela X and the Tycho supernovae. We conclude that such giant H ii regions might not in general be as easily detectable as the Vela X region. The Tycho region may just be detectable in the 0 ii, 0 iii forbidden optical lines or as a "hole" in the 21-cm emission-line profiles (the latter is already suggested in the data). These giant H ii regions last appreciably longer than the continuum radio sources within them. Since no very large H ii region is likely to be associated with a Type II supernova explosion, detection of giant H ii regions around the Galaxy could give us the frequency of Type I explosions.
Kafatos, M., Morrison, P. (1971) Fossil Stromgren Spheres from Supernova Explosions, Astrophysical Journal, 168: 195-201. doi: 10.1086/151074