SOFIA/GREAT observations of OD and OH rotational lines towards high-mass
  star forming regions
  Only recently, OD, the deuterated isotopolog of hydroxyl, OH, has become
accessible in the interstellar medium; spectral lines from both species have
been observed in the supra-Terahertz and far infrared regime. Here we study
rotational lines of OD and OH towards 13 Galactic high-mass star forming
regions, with the aim to constrain the OD abundance and infer the deuterium
fractionation of OH in their molecular envelopes. We used the Stratospheric
Observatory for Infrared Astronomy (SOFIA) to observe the $^2\Pi_{3/2}$
$J=5/2-3/2$ ground-state transition of OD at 1.3 THz ($215~\mu$m) and the
rotationally excited OH line at 1.84 THz ($163~\mu$m). We also used published
high-spectral-resolution SOFIA data of the OH ground-state transition at 2.51
THz ($119.3~\mu$m). Our results show that absorption from the $^2\Pi_{3/2}$ OD
$J=5/2-3/2$ ground-state transition is prevalent in the dense clumps
surrounding active sites of high-mass star formation. We performed detailed
radiative transfer modelling to investigate the OD abundance profile in the
inner envelope for a large fraction of our sample. Our modelling suggests that
part of the absorption arises from the denser inner parts, while the bulk of it
as seen with SOFIA originates in the outer, cold layers of the envelope for
which our constraints on the molecular abundance suggest a strong enhancement
in deuterium fractionation. We find a weak negative correlation between the OD
abundance and the bolometric luminosity to mass ratio, an evolutionary
indicator, suggesting a slow decrease of OD abundance with time. A comparison
with HDO shows a similarly high deuterium fractionation for the two species in
the cold envelopes, which is of the order of 0.48% for the best studied source,
G34.26+0.15. Our results are consistent with chemical models that favour rapid
exchange reactions to form OD in the dense cold gas.
