Ultracompact HII regions radio images

Alternative explanations are explored and found to be unlikely. The correlation of UC HII region positions with proposed spiral arms is examined and found to be well correlated only for the local spiral arm or "spur". No obvious enhancement of UC HII regions is apparent along the proposed Sagittarius and Scutum arms, probably because of inaccuracies in the kinematic distances.

We find the latitude distribution of UC HII regions to lie in the range 0.5deg<[b_FWHM_]<=0.8deg. No correlation between size and density of cometary and core-halo UC HII regions is found, consistent with the bow shock interpretation of these morphologies. Spherical and unresolved UC HII regions, however, appear to show a trend toward lower densities with increasing size, as expected for expanding HII regions.

The observed ratios of far-infrared to radio flux densities of UC HII regions lie in the range 10^3^ to >=10^5^. By applying the results of model atmospheres, it is shown that this ratio depends on spectral type, ranging from ~10^3^ for an O4 star to >=10^5^ for a B3 star. We find that many of the UC HII regions in our sample must be excited by a cluster of stars, and most probably contain significant amounts of dust.