A population of hypercompact H II regions identified from young H II regions

Abstract

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 (α 5 GHz 1.4 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 n e = 1.6 × 10 4 cm −3 , diameter diam = 0.14 pc, and emission measure EM = 1.9 × 10 7 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 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 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.

The radio spectral turnover of radio-loud quasars at $z>5$

“…Emission measure is a function of the average electron number density (n e ) and path length (L) through plasma: EM = n 2 e × L. Assuming the path length (L) is the same with the source size (we assume each component of J1146+4037 has a size of 2.5 pc), we calculate the electron number density n e of 1.22 +0.11 −0.11 × 10 4 , 1.58 +0.26 −0.24 × 10 4 , and 3.59 +3.88 −2.12 × 10 4 cm −3 for J0131−0321 and the two components of J1146+4037, respectively. The derived electron number densities are consistent with that (a mean n e of 1.6 × 10 4 cm −3 ) of a sample of 114 young H ii regions in the Galaxy (Yang et al 2021). The n e can be up to 10 5 cm −3 for hyper-compact H ii regions (e.g., Murphy et al 2010b).…”

The SEDIGISM survey: A search for molecular outflows

“…The outflow wing detection rate in UC H ii regions can be as high as 80% (70/87) for clumps associated with UC H ii regions and masers (H 2 O: 39/50; CH 3 OH: 57/69), and it drops to 65% (48/74) for clumps with UC H ii regions without any maser emission. The detection fraction can even rise to 100% (5/5) for clumps associated with hyper-compact (HC) H ii regions (Yang et al 2019(Yang et al , 2021, however, we note that the sample size is small. This suggests that the outflow detection rate appears to peak in the pre-UC H ii stage (i.e., HC H ii regions, maser-associated UC H ii regions), as stated in Paper I, and then it starts to decrease in the late UC H ii regions phase (no-maser-associated UC H ii regions).…”

The SEDIGISM survey: a search for molecular outflows

“…The outflow wing detection rate in UC H ii regions can be as high as 80% (70/87) for clumps associated with UC H ii regions and masers (H 2 O: 39/50; CH 3 OH: 57/69), and it drops to 65% (48/74) for clumps with UC H ii regions without any maser emission. The detection fraction can even rise to 100% (5/5) for clumps associated with hyper-compact (HC) H ii regions (Yang et al 2019(Yang et al , 2021. However, we note that the sample size is small.…”