Ultra- and hyper-compact H ii regions at 20 GHz.

We present radio and infrared observations of four hyper-compact H ii regions and four ultra-compact H ii regions in the southern Galactic plane. These objects were selected from a blind survey for ultra-compact H ii regions using data from two new radio surveys of the southern sky: the Australia Telescope 20 GHz survey and the second epoch Molonglo Galactic Plane Survey at 843 MHz. To our knowledge, this is the first blind radio survey for hyper- and ultra-compact H ii regions. We have followed up these sources with the Australia Telescope Compact Array to obtain H70α recombination line measurements, higher resolution images at 20 GHz and flux density measurements at 30, 40 and 95 GHz. From this, we have determined sizes and recombination line temperatures as well as modelling the spectral energy distributions to determine emission measures. We have classified the sources as hyper-compact or ultra-compact on the basis of their physical parameters, in comparison with benchmark parameters from the literature. Several of these bright, compact sources are potential calibrators for the low-frequency instrument (30–70 GHz) and the 100 GHz channel of the high-frequency instrument of the Planck satellite mission. They may also be useful as calibrators for the Australia Telescope Compact Array, which lacks good non-variable primary flux calibrators at higher frequencies and in the Galactic plane region. Our spectral energy distributions allow the flux densities within the Planck bands to be determined, although our high-frequency observations show that several sources have excess emission at 95 GHz (3 mm) that cannot be explained by current models. [ABSTRACT FROM AUTHOR]