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Context. The derived physical parameters for young HII regions are normallydetermined assuming the emission region to be optically thin. However, thisassumption is unlikely to hold for young HII regions such as hyper-compactHII(HCHII) and ultra-compact HII(UCHII) regions and leads to theunderestimation of their properties. This can be overcome by fitting the SEDsover a wide range of radio frequencies.  Aims. The two primary goals of this study are (1) to determine the physicalproperties of young HII regions from radio SEDs in the search for potentialHCHII regions, and (2) to use these physical properties to investigate theirevolution.  Method. We used the Karl G. Jansky Very Large Array (VLA) to observe theX-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 ourobservations with VLA archival data and construct SEDs in the range of 1-26 GHzand model them assuming an ionization-bounded HII region with uniform density.  Results. Our sample has a mean electron density of ne=1.6E4cm^{-3}, diameterdiam=0.14pc, and emission measure EM = 1.9E7pc*cm^{-6}. We identify 16 HCHIIregion candidates and 8 intermediate objects between the classes of HCHII andUCHII regions. The ne, diam, and EM change as expected, but the Lyman continuumflux is relatively constant over time. We find that about 67% ofLyman-continuum photons are absorbed by dust within these HII regions and thedust absorption fraction tends to be more significant for more compact andyounger HII regions.  Conclusion. Young HII regions are commonly located in dusty clumps; HCHIIregions and intermediate objects are often associated with various masers,outflows, broad radio recombination lines, and extended green objects, and theaccretion at the two stages tends to be quickly reduced or halted.

A population of hypercompact HII regions identified from young HII regions