A search for hypercompact HII regions in the Galactic Plane

TL;DR

This study conducted the largest unbiased search for hypercompact HII regions in the Galactic Plane, revealing that current definitions may be inadequate and that many ultracompact HII regions exhibit positive spectral indices, indicating a mix of optical depths.

Contribution

It challenges the canonical definition of hypercompact HII regions and highlights the prevalence of positive spectral indices among ultracompact HII regions, suggesting a need for revised classification criteria.

Findings

None of the identified HII regions meet the canonical HCHII definition at 5 GHz.

About half of ultracompact HII regions have positive spectral indices, indicating mixed optical depths.

Positive spectrum HII regions are more luminous and associated with more massive stars.

Abstract

We have carried out the largest and most unbiased search for hypercompact (HC) HII 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 HII regions with positive spectral indices. 120 positive spectrum HII regions have been identified from a total sample of 534 positive spectral index radio sources. None of these HII regions, including the known HCHII regions recovered in our search, fulfills the canonical definition of an HCHII region at 5 GHz. We suggest that the current canonical definition of HCHII 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) HII region surveys, we find that roughly half of detected UCHII 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 HII regions are statistically more luminous and possess higher Lyman continuum fluxes than HII regions with flat or negative indices. Positive spectrum HII 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 HII regions.