SOFIA observations of far-infrared hydroxyl emission toward classical ultracompact HII/OH maser regions

TL;DR

This study uses SOFIA to observe far-infrared hydroxyl emission in dense, warm regions around ultracompact HII regions, providing new insights into the physical conditions of these environments.

Contribution

First observations of specific far-infrared OH lines in ultracompact HII regions using SOFIA, combined with non-LTE modeling to interpret the emission.

Findings

Detected OH lines in dense, warm environments

Distinguished between low-abundance envelopes and high-abundance sources

Provided constraints on physical conditions of the regions

Abstract

The hydroxyl radical (OH) is found in various environments within the interstellar medium (ISM) of the Milky Way and external galaxies, mostly either in diffuse interstellar clouds or in the warm, dense environments of newly formed low-mass and high-mass stars, i.e, in the dense shells of compact and ultracompact HII regions (UCHIIRs). Until today, most studies of interstellar OH involved the molecule's radio wavelength hyperfine structure (hfs) transitions. These lines are generally not in LTE and either masing or over-cooling complicates their interpretation. In the past, observations of transitions between different rotational levels of OH, which are at far-infrared wavelengths, have suffered from limited spectral and angular resolution. Since these lines have critical densities many orders of magnitude higher than the radio wavelength ground state hfs lines and are emitted from levels with more than 100 K above the ground state, when observed in emission, they probe very dense and warm material. We probe the warm and dense molecular material surrounding the UCHIIR/OH maser sources W3(OH), G10.62-0.39 and NGC 7538 IRS1 by studying the $^2\Pi_{{1/2}}, J = {3/2} - {1/2}$ rotational transition of OH in emission and, toward the last source also the molecule's $^2\Pi_{3/2}, J = 5/2 - 3/2$ ground-state transition in absorption. We used the Stratospheric Observatory for Infrared Astronomy (SOFIA) to observe these OH lines, which are near 1.84 THz ($163 \mu$m) and 2.51 THz ($119.3 \mu$m). We clearly detect the OH lines, some of which are blended with each other. Employing non-LTE radiative transfer calculations we predict line intensities using models of a low OH abundance envelope versus a compact, high-abundance source corresponding to the origin of the radio OH lines.