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

¹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: ayyang@mpifr-bonn.mpg.de
² Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
³ Centre for Astrophysics Research, School of Physics Astronomy & Mathematics, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
⁴ CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁵ University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, PR China
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China

Received: 8 June 2020
Accepted: 13 November 2020

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_1.4 GHz^5 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 × 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 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.