Physical characterization and modeling of candidate Hyper-Compact HII Regions

TL;DR

This study characterizes five candidate hyper-compact HII regions using VLA observations, modeling their physical properties to distinguish between ultra-compact HII regions and other phenomena like jets or winds.

Contribution

It provides detailed measurements and modeling of candidate HC HII regions, estimating their sizes and densities to improve understanding of early high-mass star formation stages.

Findings

Most sources are consistent with small, weak UC HII regions.

Some sources may be jets or stellar winds.

G40.28-0.22 is the strongest candidate for HC HII.

Abstract

Hypercompact HII regions (HC) are regions of ionized gas associated with the early stages of high-mass star formation. With the aim of better understanding their characteristics, we studied five candidate HC HII regions. Here, we present observations with the Jansky Very Large Array (VLA) at 2 and 6 cm, with angular resolutions in the range of $\sim$1 -- 3\arcsec and report the images of the detected sources and the measured parameters. In addition, we explore several possible scenarios, considering the regions as both uniform and non-uniform spheres, and as winds, both spherical and collimated. In most cases, the sources were unresolved, but by applying the models, we estimate that their sizes vary in a range of 0.3 to 3.7 mpc while their electron densities are in the range of $1.3 \times 10^{5}$ to $2.4 \times 10^{6}$ cm$^{-3}$, indicating that most sources are consistent with small, weak UC HII regions, although a few remain viable candidates for HC HII regions, with G40.28$-$0.22 as the strongest case. We do not rule out the possibility that some sources are jets or stellar winds.