Too large and overlooked? Extended free–free emission towards massive star formation regions
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We present Australia Telescope Compact Array observations towards six massive star formationregions, which, from their strong 24 GHz continuum emission but no compact 8 GHzcontinuum emission, appeared good candidates for hypercompact HII regions. However, theproperties of the ionized gas derived from the 19 to 93 GHz continuum emission and H70a+ H57a radio recombination line data show the majority of these sources are, in fact, regionsof spatially extended, optically thin free–free emission. These extended sources were missedin the previous 8 GHz observations due to a combination of spatial filtering, poor surfacebrightness sensitivity and primary beam attenuation.We consider the implications that a significant number of these extended HII regions mayhave been missed by previous surveys of massive star formation regions. If the originalsample of 21 sources is representative of the population as a whole, the fact that six containpreviously undetected extended free–free emission suggests a large number of regions havebeen 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 ~0.5 Jy) from free–free emission has implications for dust masses derived from sub-mm flux densities. The largespatial scales probed by single-dish telescopes, which do not suffer from spatial filtering, areparticularly susceptible and dust masses may be overestimated by up to a factor of ~2.
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