Challenging shock models with SOFIA OH observations in the high-mass star-forming region Cepheus A

Context. OH is a key molecule in H2 O chemistry, a valuable tool for probing physical conditions, and an important contributor to the cooling of shock regions around high-mass protostars. OH participates in the re-distribution of energy from the protostar towards the surrounding Interstellar Medium.Aims. 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.Methods. We present spectrally resolved observations of OH towards the protostellar outflows region of Cepheus A with the GREAT spectrometer onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope. Three triplets were observed at 1834.7 GHz, 1837.8 GHz, and 2514.3 GHz (163.4 μ m, 163.1 μ m between the 2 Π1/2 J = 1/2 states, and 119.2 μ m, a ground transition between the 2 Π3/2 J = 3/2 states), at angular resolutions of 16.3, 16.3, and 11.9, respectively. We also present the CO (16–15) spectrum at the same position. We compared the integrated intensities in the redshifted wings to the results of shock models.Results. The two OH triplets near 163 μ m are detected in emission, but with blending hyperfine structure unresolved. Their profiles and that of CO (16–15) can be fitted by a combination of two or three Gaussians. The observed 119.2 μ m 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.Conclusions. 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. Overall, our comparisons suggest that all the observations might be consistently fitted by a J-type shock model with a high pre-shock density (n H > 105 cm-3 ), a high shock velocity (νs ≳ 25 km s-1 ), and with a filling factor of the order of unity. Such a high pre-shock density is generally found in shocks associated to high-mass protostars, contrary to low-mass ones.