A population of hypercompact HII regions identified from young HII regions
Context. The derived physical parameters for young H II regions are normally determined assuming the emission region to be optically thin. However, this assumption is unlikely to hold for young H II regions such as hyper-compact H II (HC H II) and ultra-compact H II (UC H II) regions and leads to 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 H II regions from radio SEDs in the search for potential HC H II regions, and (2) to use these physical properties to investigate their evolution. Methods. 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 H II regions with rising-spectra (α1.4 GHz5 GHz>0). 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 H II region with uniform density. Results. Our sample has a mean electron density of ne = 1.6 × 104 cm-3, diameter diam = 0.14 pc, and emission measure EM = 1.9 × 107 pc cm-6. We identify 16 HC H II region candidates and 8 intermediate objects between the classes of HC H II and UC H II 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 H II regions and the dust absorption fraction tends to be more significant for more compact and younger H II regions. Conclusions. Young H II regions are commonly located in dusty clumps; HC H 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.