Physical characterization and modeling of candidate Hyper-Compact HII Regions

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.

Accepted for publication in Revista Mexicana de Astronomía y Astrofísica (RMxAA), Vol. XX, 2026