Recombination Lines and Molecular Gas from Hypercompact HII regions in W51 A

TL;DR

This study characterizes hypercompact HII regions in W51 A using high-resolution radio observations, revealing their sizes, densities, ionization sources, and gas dynamics, and proposing two types of hypercompact HII regions based on their properties.

Contribution

It provides detailed measurements and analysis of hypercompact HII regions, introducing a classification into two types based on size, density, and evolutionary stage.

Findings

Most sources ionized by early B-type stars.

Ionized gas shows outflow in all cases.

Evidence of expansion and infall motions in some regions.

Abstract

We present a detailed characterization of the population of compact radio-continuum sources in W51 A using subarcsecond VLA and ALMA observations. We analyzed their 2-cm continuum, the recombination lines (RL's) H77$\alpha$ and H30$\alpha$, and the lines of $\rm H_{2}CO(3_{0,3}-2_{0,2})$, $\rm H_{2}CO(3_{2,1}-2_{2,0})$, and $\rm SO(6_{5}-5_{4})$. We derive diameters for 10/20 sources in the range $D \sim 10^{-3}$ to $\sim 10^{-2}$ pc, thus placing them in the regime of hypercompact HII regions (HC HII's). Their continuum-derived electron densities are in the range $n_{\rm e} \sim 10^4$ to $10^5$ cm$^{-3}$, lower than typically considered for HC HII's. We combined the RL measurements and independently derived $n_{\rm e}$, finding the same range of values but significant offsets for individual measurements between the two methods. We found that most of the sources in our sample are ionized by early B-type stars, and a comparison of $n_{\rm e}$ vs $D$ shows that they follow the inverse relation previously derived for ultracompact (UC) and compact HII's. When determined, the ionized-gas kinematics is always (7/7) indicative of outflow. Similarly, 5 and 3 out of the 8 HC HII's still embedded in a compact core show evidence for expansion and infall motions in the molecular gas, respectively. We hypothesize that there could be two different types of $hypercompact$ ($D< 0.05$ pc) HII regions: those that essentially are smaller, expanding UC HII's; and those that are also $hyperdense$ ($n_{\rm e} > 10^6$ cm$^{-3}$), probably associated with O-type stars in a specific stage of their formation or early life.