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L1157-B1 is the brightest shocked region of the large-scale molecularoutflow, considered the prototype of chemically rich outflows, being the ideallaboratory to study how shocks affect the molecular gas. Several deuteratedmolecules have been previously detected with the IRAM 30m, most of them formedon grain mantles and then released into the gas phase due to the shock. We aimto observationally investigate the role of the different chemical processes atwork that lead to formation the of DCN and test the predictions of the chemicalmodels for its formation. We performed high-angular resolution observationswith NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuteratedfraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arisingfrom L1157-B1 shock. Dfrac(HCN) is ~4x10$^{-3}$ and given the uncertainties, wedid not find significant variations across the bow-shock. Contrary to HDCO,whose emission delineates the region of impact between the jet and the ambientmaterial, DCN is more widespread and not limited to the impact region. This isconsistent with the idea that gas-phase chemistry is playing a major role inthe deuteration of HCN in the head of the bow-shock, where HDCO is undetectedas it is a product of grain-surface chemistry. The spectra of DCN and H13CNmatch the spectral signature of the outflow cavity walls, suggesting that theiremission result from shocked gas. The analysis of the time dependent gas-grainchemical model UCL-CHEM coupled with a C-type shock model shows that theobserved Dfrac(HCN) is reached during the post-shock phase, matching thedynamical timescale of the shock. Our results indicate that the presence of DCNin L1157-B1 is a combination of gas-phase chemistry that produces thewidespread DCN emission, dominating in the head of the bow-shock, andsputtering from grain mantles toward the jet impact region.

The L1157-B1 astrochemical laboratory: testing the origin of DCN