A population of hypercompact HII regions identified from young HII regions

CONTEXT: The derived physical parameters for young H\(\tiny{II}\) regions are normally determined assuming the emission region to be optically-thin. However, this is unlikely to hold for young H\(\tiny{II}\) regions such as Hyper-compact H\(\tiny{II}\) (HC H\(\tiny{II}\)) and Ultra-compact H\(\tiny{II}\) (UC H\(\tiny{II}\)) 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: Two primary goals are (1) determining physical properties from radio SEDs and finding potential HC H\(\tiny{II}\) regions; (2) using these physical properties to investigate their evolution. METHODS: We used Karl G. Jansky Very Large Array (VLA) to make observations of X-band and K-band with angular-resolutions of ~ 1:7′′ and ~ 0:7′′, respectively, toward 114 H\(\tiny{II}\) regions with rising-spectra \(α{5GHz\choose 1.4GHz} > 0\). We complement our observations with VLA archival data and construct SEDs between 1-26 GHz and model them assuming an ionisation-bounded H\(\tiny{II}\) region with uniform density. RESULTS: The sample has a mean electron density of n\(_e\) = 1.6 x 10\(^4\) cm\(^{-3}\), diameter diam = 0.14 pc, and emission measure EM = 1.9 x 10\(^7\) pc cm\(^{-6}\). We identify 16 HC H\(\tiny{II}\) region candidates and 8 intermediate objects between the classes of HC H\(\tiny{II}\) and UC H\(\tiny{II}\) regions. The n\(_e\), diam, and EM change as expected, however, the Lyman continuum flux is relatively constant over time. We find that about 67% of Lyman continuum photons are absorbed by dust within these H\(\tiny{II}\) regions and the dust absorption fraction tends to be more significant for more compact and younger H\(\tiny{II}\) regions. CONCLUSIONS: Young H\(\tiny{II}\) regions are commonly located in dusty clumps; HC H\(\tiny{II}\) 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.