SCOTCH – search for clandestine optically thick compact H ii regions: II

Author:

Patel A L1ORCID,Urquhart J S1ORCID,Yang A Y23,Moore T4,Thompson M A5,Menten K M6,Csengeri T7

Affiliation:

1. Centre for Astrophysics and Planetary Science, University of Kent , Canterbury CT2 7NH , UK

2. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District , Beijing 100101 , P. R. China

3. Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences , A20 Datun Road, Chaoyang District, Beijing 100101 , P. R. China

4. Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park , 146 Brownlow Hill, Liverpool L3 5RF , UK

5. School of Physics and Astronomy, University of Leeds , Leeds LS2 9JT , UK

6. Max Planck Institute for Radio Astronomy , Auf dem Hügel 69, D-53121 Bonn , Germany

7. Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS , B18N, allée Geoffroy Saint-Hilaire, F-33615 Pessac , France

Abstract

ABSTRACT In this study we present 18$-$24 GHz and high-angular-resolution (0.5 arcsec) radio wavelength Australia Telescope Compact Array follow-up observations towards a sample of 39 HC H ii region candidates. These objects, taken from a sample hosting 6.7 GHz methanol masers, were chosen due to the compact and optically thick nature of their continuum emission. We have detected 27 compact radio sources and constructed their spectral energy distributions over the 5–24 GHz range to determine the young H ii region’s physical properties, i.e. diameter, electron density ${\it n}_{\mbox{e}}$, emission measure, Lyman continuum flux ${\it N}_{\mbox{Ly}}$, and turnover frequency ${\nu }_{\mbox{t}}$. The flux measurements are fitted for 20 objects assuming an ionization-bounded H ii region with uniform density model. For the remaining seven objects that lack constraints spanning both their optically thick and thin regimes, we utilize relations from the literature to determine their physical properties. Comparing these determined parameters with those of known hypercompact (HC) and ultracompact (UC) H ii regions, we have identified 13 HC H ii regions, six intermediate objects that fall between HC H ii and UC H ii regions, six UC H ii regions and one radio jet candidate which increases the known population of HC H ii regions by $\sim$50 per cent. All the young and compact H ii regions are embedded in dusty and dense clumps and $\sim$80 per cent of the HC H ii regions identified in this work are associated with various maser species (CH$_3$OH, H$_2$O, and OH). Four of our radio sources remain optically thick at 24 GHz; we consider these to be among the youngest HC H ii regions.

Funder

National Key Research and Development Program of China

National Natural Science Foundation of China

Science and Technology Facilities Council

Publisher

Oxford University Press (OUP)

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