Ultra- and Hyper-compact HII regions at 20 GHz
  We present radio and infrared observations of 4 hyper-compact HII regions and
4 ultra-compact HII regions in the southern Galactic plane. These objects were
selected from a blind survey for UCHII regions using data from two new radio
surveys of the southern sky; the Australia Telescope 20 GHz survey (AT20G) and
the 2nd epoch Molonglo Galactic Plane Survey (MGPS-2) at 843 MHz. To our
knowledge, this is the first blind radio survey for hyper- and ultra-compact
HII regions.
  We have followed up these sources with the Australia Telescope Compact Array
to obtain H70-alpha 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 modeling 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 can not be
explained by current models.
