A search for hypercompact HII regions in the Galactic Plane
  We have carried out the largest and most unbiased search for hypercompact
(HC) HII regions. Our method combines four interferometric radio continuum
surveys (THOR, CORNISH, MAGPIS and White2005) with far-infrared and sub-mm
Galactic Plane surveys to identify embedded HII regions with positive spectral
indices. 120 positive spectrum HII regions have been identified from a total
sample of 534 positive spectral index radio sources. None of these HII regions,
including the known HCHII regions recovered in our search, fulfills the
canonical definition of an HCHII region at 5 GHz. We suggest that the current
canonical definition of HCHII regions is not accurate and should be revised to
include a hierarchical structure of ionized gas that results in an extended
morphology at 5 GHz. Correlating our search with known ultracompact (UC) HII
region surveys, we find that roughly half of detected UCHII regions have
positive spectral indices, instead of more commonly assumed flat and optically
thin spectra. This implies a mix of optically thin and thick emission and has
important implications for previous analyses which have so far assumed
optically thin emission for these objects. Positive spectrum HII regions are
statistically more luminous and possess higher Lyman continuum fluxes than HII
regions with flat or negative indices. Positive spectrum HII regions are thus
more likely to be associated with more luminous and massive stars. No
differences are found in clump mass, linear diameter or luminosity-to-mass
ratio between positive spectrum and non-positive spectrum HII regions.
