SIMBAD references

We have used the Australia Telescope Compact Array (ATCA) to make observations of a sample of eight young ultra-compact HII regions, selected on the basis that they have associated class II methanol maser emission. We have made observations sensitive to both compact and extended structures and find both to be present in most sources. The scale of the extended emission in our sample is in general less than that observed towards samples based on IRAS properties, or large single-dish flux densities. Our observations are consistent with a scenario where extended and compact radio continuum emission co-exists within HII regions for a significant period of time.

We suggest that these observations are consistent with a model where HII evolution takes place within hierarchically structured molecular clouds. This model, which is the subject of an upcoming companion paper by Shabala et al., addresses both the association between compact and extended emission and the ultra-compact HII region lifetime problem.