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