A search for hypercompact H ii regions in the Galactic Plane. Issue 2 (10th November 2018)

• Record Type: Journal Article
• Authors: Yang, A Y; Thompson, M A; Tian, W W; Bihr, S; Beuther, H; Hindson, L
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 482:Issue 2(2019)
• Page Start: 2681
• Page End: 2696
• Publication Date: 2018-11-10
• Subjects: ISM: H ii regions -- infrared: ISM -- radio continuum: ISM -- submillimetre: ISM; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty2811
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We have carried out the largest and most unbiased search for hypercompact (HC) H ii regions. Our method combines four interferometric radio continuum surveys (THOR, CORNISH, MAGPIS, and White2005) with far-infrared and sub-mm Galactic Plane surveys to identify embedded H ii regions with positive spectral indices; 120 positive spectrum H ii regions have been identified from a total sample of 534 positive spectral index radio sources. None of these H ii regions, including the known HC H ii regions recovered in our search, fulfills the canonical definition of an HC H ii region at 5 GHz. We suggest that the current canonical definition of HC H ii regions is not accurate and should be revised to include a hierarchical structure of ionized gas that results in an extended morphology at 5 GHz. Correlating our search with known ultracompact (UC) H ii region surveys, we find that roughly half of detected UC H ii regions have positive spectral indices, instead of more commonly assumed flat and optically thin spectra. This implies a mix of optically thin and thick emission and has important implications for previous analyses which have so far assumed optically thin emission for these objects. Positive spectrum H ii regions are statistically more luminous and possess higher Lyman continuum fluxes than H ii regions with flat or negative indices. Positive spectrum H ii regions are thus more likely to be associated with more luminous and massive stars. No differences are found in clump mass, linear diameter, or luminosity-to-mass ratio between positive spectrum and non-positive spectrum H ii regions.