A VLA Study of Ultracompact and Hypercompact H II Regions from 0.7 to 3.6 cm

TL;DR

This study uses high-resolution multi-frequency VLA observations to analyze the physical conditions and structures of ultracompact and hypercompact H II regions in massive star formation areas, focusing on radio recombination lines and continuum emission.

Contribution

It provides detailed multi-frequency radio recombination line data and continuum observations, revealing pressure broadening effects and structural details of hypercompact H II regions.

Findings

Pressure broadening significantly affects line widths.

Good correspondence between radio continuum and dust emission.

Identification of complex velocity structures in H II regions.

Abstract

We report multi-frequency Very Large Array observations of three massive star formation regions (MSFRs) containing radio continuum components that were identified as broad radio recombination line (RRL) sources and hypercompact (HC) H II region candidates in our previous H92alpha and H76alpha study: G10.96+0.01 (component W), G28.20-0.04 (N), and G34.26+0.15 (B). An additional HC H II region candidate, G45.07+0.13, known to have broad H66alpha and H76alpha lines, small size, high electron density and emission measure, was also included. We observed with high spatial resolution (0.9" to 2.3") the H53alpha, H66alpha, H76alpha, and H92alpha RRLs and the radio continuum at the corresponding wavelengths (0.7 to 3.6 cm). The motivation for these observations was to obtain RRLs over a range of principal quantum states to look for signatures of pressure broadening and macroscopic velocity structure. We find that pressure broadening contributes significantly to the line widths, but it is not the sole cause of the broad lines. We compare radio continuum and dust emission distributions and find a good correspondence. We also discuss maser emission and multi-wavelength observations reported in the literature for these MSFRs.