A population of hypercompact HII regions identified from young HII
  regions
  Context. The derived physical parameters for young HII regions are normally
determined assuming the emission region to be optically thin. However, this
assumption is unlikely to hold for young HII regions such as hyper-compact
HII(HCHII) and ultra-compact HII(UCHII) regions and leads to the
underestimation of their properties. This can be overcome by fitting the SEDs
over a wide range of radio frequencies.
  Aims. The two primary goals of this study are (1) to determine the physical
properties of young HII regions from radio SEDs in the search for potential
HCHII regions, and (2) to use these physical properties to investigate their
evolution.
  Method. We used the Karl G. Jansky Very Large Array (VLA) to observe the
X-band and K-band with angular resolutions of ~1.7" and ~0.7", respectively,
toward 114 HII regions with rising-spectra between 1-5 GHz. We complement our
observations with VLA archival data and construct SEDs in the range of 1-26 GHz
and model them assuming an ionization-bounded HII region with uniform density.
  Results. Our sample has a mean electron density of ne=1.6E4cm^{-3}, diameter
diam=0.14pc, and emission measure EM = 1.9E7pc*cm^{-6}. We identify 16 HCHII
region candidates and 8 intermediate objects between the classes of HCHII and
UCHII regions. The ne, diam, and EM change as expected, but the Lyman continuum
flux is relatively constant over time. We find that about 67% of
Lyman-continuum photons are absorbed by dust within these HII regions and the
dust absorption fraction tends to be more significant for more compact and
younger HII regions.
  Conclusion. Young HII regions are commonly located in dusty clumps; HCHII
regions and intermediate objects are often associated with various masers,
outflows, broad radio recombination lines, and extended green objects, and the
accretion at the two stages tends to be quickly reduced or halted.
