2008ApJ...681.1327S

Here we address the question of whether the ionized shells associated with giant H II regions can be progenitors of the larger H I shell-like objects found in the Milky Way and other spiral and dwarf irregular galaxies. We use for our analysis a sample of 12 H II shells presented recently by Relaño et al. We calculate the evolutionary tracks that these shells would have if their expansion is driven by multiple supernovae explosions from the parental stellar clusters. We find, contrary to Relaño et al., that the evolutionary tracks of their sample H II shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. We conclude that H II shells found inside giant H II regions may represent the progenitors of small or intermediate H I shells; however, they cannot evolve into the largest H I objects unless, aside from the multiple supernovae explosions, an additional energy source contributes to their expansion.