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