2009MNRAS.399..861L

We present Australia Telescope Compact Array observations towards six massive star formation regions, which, from their strong 24GHz continuum emission but no compact 8GHz continuum emission, appeared good candidates for hypercompact Hii regions. However, the properties of the ionized gas derived from the 19 to 93GHz continuum emission and H70α + H57α radio recombination line data show the majority of these sources are, in fact, regions of spatially extended, optically thin free-free emission. These extended sources were missed in the previous 8GHz observations due to a combination of spatial filtering, poor surface brightness sensitivity and primary beam attenuation.

We consider the implications that a significant number of these extended Hii regions may have been missed by previous surveys of massive star formation regions. If the original sample of 21 sources is representative of the population as a whole, the fact that six contain previously undetected extended free-free emission suggests a large number of regions have been mis-classified. Rather than being very young objects prior to UCHii region formation, they are, in fact, associated with extended Hii regions and thus significantly older. In addition, inadvertently ignoring a potentially substantial flux contribution (up to ∼0.5Jy) from free-free emission has implications for dust masses derived from sub-mm flux densities. The large spatial scales probed by single-dish telescopes, which do not suffer from spatial filtering, are particularly susceptible and dust masses may be overestimated by up to a factor of ∼2.