Evolution of H II regions in hierarchically structured molecular clouds

We present observations of the H91α recombination line emission towards a sample of nine H II regions associated with 6.7-GHz methanol masers, and report arcsecond-scale emission around compact cores. We derive physical parameters for our sources, and find that although simple hydrostatic models of region evolution reproduce the observed region sizes, they significantly underestimate emission measures. We argue that these findings are consistent with young source ages in our sample, and can be explained by existence of density gradients in the ionized gas. © 2006 RAS.