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

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.

10 pages, 7 figures, 5 tables

References in corpus (11)

• GREAT: the SOFIA high-frequency heterodyne instrument
• SiO line emission from C-type shock waves : interstellar jets and outflows
• GREAT/SOFIA atmospheric calibration
• Low velocity shocks: signatures of turbulent dissipation in diffuse irradiated gas
• The ALMA view of the protostellar system HH212 - The wind, the cavity, and the disk
• The jet and the disk of the HH 212 low-mass protostar imaged by ALMA: SO and SO2 emission
• A new probe of magnetic fields during high-mass star formation: Zeeman splitting of 6.7 GHz methanol masers
• Discovery of superthermal hydroxyl (OH) in the HH211 outflow
• Shockingly low water abundances in Herschel / PACS observations of low-mass protostars in Perseus
• Shocks in dense clouds. IV. Effects of grain-grain processing on molecular line emission
• Water emission from the high-mass star-forming region IRAS 17233-3606. High water abundances at high velocities

Cited by in corpus (3)

• ALMA Observations of the massive molecular outflow G331.512-0.103 II: physical properties, kinematics, and geometry modeling
• Velocity-resolved high-J CO emission from massive star-forming clumps
• SOFIA FIFI-LS spectroscopy of DR21 Main: energetics of the spatially-resolved outflow from a high-mass protostar