Too large and overlooked? Extended free–free emission towards massive star formation regions |
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| Authors: | S N Longmore M G Burton E Keto S Kurtz A J Walsh |
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| Institution: | Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA;School of Physics, University of New South Wales, 2052, Sydney, Australia;Australia Telescope National Facility, CSIRO, Epping 1710, Sydney, Australia;CRyA, Universidad Nacional Autónoma de México, Apartado Postal 3-72, 58089, Morelia, Michoacán, México;Centre for Astronomy, James Cook University, Townsville, QLD 4811 Australia |
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| Abstract: | We present Australia Telescope Compact Array observations towards six massive star formation regions, which, from their strong 24 GHz continuum emission but no compact 8 GHz continuum emission, appeared good candidates for hypercompact H ii regions. However, the properties of the ionized gas derived from the 19 to 93 GHz 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 8 GHz 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 H ii 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 UCH ii region formation, they are, in fact, associated with extended H ii 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 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. |
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| Keywords: | stars: early-type stars: evolution stars: formation ISM: evolution H ii regions radio continuum: ISM radio lines: ISM |
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