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

Abstract

Context. OH is a key molecule in H 2 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 µm, 163.1 µm between the 2 Π 1/2 J = 3/2 and J = 1/2 states, and 119.2 µm, a ground transition between the 2 Π 3/2 J = 5/2 and 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 µm are detected in emission, but with blending hyperfine structure unresolved. Their profiles and that of can be fitted by a combination of two or three Gaussians. The observed 119.2 µm 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 > 10 5 cm −3 ), a high shock velocity ( s 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.

Feedback from young stars, the molecular signature of shocks and outflows

“…Observationnally, these new tools will allow to interpret observations of shocks closer to the protostar ( [21], [22], [23], [24], [25],...) including accretion shocks (where the material falls on the disk - [26], [27], [28],...). They will also provide support to interpret observations from high-mass protostellar regions ( [29], [30], [31], [32],...), and also from later feedback stages ( [33], [34], [35],...). These new shock models are fed with more input parameters, and require more observational constraints.…”

ALMA Observations of the Massive Molecular Outflow G331.512-0.103. II. Physical Properties, Kinematics, and Geometry Modeling

“…They conclude that the SiO rotational emission spectra can be modeled in shocks located in high-mass star forming regions, using the tools developed for more quiescent low-mass star forming regions, but using higher values for pre-shock densities. Gusdorf et al (2016) uses the shock model for Cepheus A, a massive star nursery, but for CO and OH line observations.…”