A population of hypercompact H II regions identified from young H II 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 (HC HII) and ultra-compact HII (UC HII) 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 HII regions from radio SEDs in the search for potential HC HII 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 similar to 1.7 and similar to 0.7 '', respectively, toward 114 HII regions with rising-spectra (alpha(5GHz)(1.4GHz)>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 HII region with uniform density.Results. Our 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 HII region candidates and 8 intermediate objects between the classes of HC HII and UC HII regions. The n(e), 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.Conclusions. Young HII regions are commonly located in dusty clumps; HC HII 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.