Challenging shock models with SOFIA OH observations in the high-mass
  star-forming region Cepheus A
  OH is a key molecule in H2O chemistry, a valuable tool for probing physical
conditions, and an important contributor to the cooling of shock regions. OH
participates in the re-distribution of energy from the protostar towards the
surrounding ISM. Our aim is to assess the origin of the OH emission from the
Cepheus A massive star-forming region and to constrain the physical conditions
prevailing in the emitting gas. We thus want to probe the processes at work
during the formation of massive stars. We present spectrally resolved
observations of OH towards the outflows of Cepheus A with the GREAT
spectrometer onboard the SOFIA telescope. Three triplets were observed at
1834.7 GHz, 1837.8 GHz, and 2514.3 GHz (163.4, 163.1, and 119.2 microns), at
angular resolutions of 16.3", 16.3", and 11.9", respectively. We present the CO
(16-15) spectrum at the same position. We compared the integrated intensities
in the redshifted wings to shock models. The two triplets near 163 microns are
detected in emission with blending hyperfine structure unresolved. Their
profiles and that of CO can be fitted by a combination of 2 or 3 Gaussians. The
observed 119.2 microns triplet is seen in absorption, since its blending
hyperfine structure is unresolved, but with three line-of-sight components and
a blueshifted emission wing consistent with that of the other lines. The OH
line wings are similar to those of CO, suggesting that they emanate from the
same shocked structure. Under this common origin assumption, the observations
fall within the model predictions and within the range of use of our model only
if we consider that four shock structures are caught in our beam. Our
comparisons suggest that the observations might be consistently fitted by a
J-type model with nH > 1e5 cm-3, v > 20 km/s, and with a filling factor of ~1.
Such a high density is generally found in shocks associated to high-mass
protostars.
