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

TL;DR

This study uses SOFIA OH observations to investigate shock processes in the high-mass star-forming region Cepheus A, revealing that the emission originates from multiple shock structures consistent with J-type shocks at high densities.

Contribution

First spectrally resolved OH observations of Cepheus A's outflows, providing new insights into shock conditions and challenging existing shock models in high-mass star formation regions.

Findings

OH emission lines are consistent with multiple shock structures.

Observations fit well with high-density J-type shock models.

OH and CO line profiles suggest common shock origins.

Abstract

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.