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

摘要

Context. L1157-B1 is the brightest shocked region of the large-scale molecular outflow. It is considered the prototype of the so-called chemically rich active outflows, being the perfect laboratory to study how shocks affect the molecular gas content. Specifically, several deuterated molecules have previously been detected with the IRAM 30?m telescope, most of them formed on grain mantles and then released into the gas phase due to the passage of the shock. Aims. We aim to observationally investigate the role of the different chemical processes at work that lead to formation of the DCN and compare it with HDCO, the two deuterated molecules imaged with an interferometer, and test the predictions of the chemical models for their formation. Methods. We performed high-angular-resolution observations toward L1157-B1 with the IRAM NOEMA interferometer of the DCN?(2–1) and H ~(13) CN?(2–1) lines to compute the deuterated fraction, D _(frac) (HCN), and compare it with previously reported D _(frac) of other molecular species. Results. We detected emission of DCN?(2–1) and H ~(13) CN?(2–1) arising from L1157-B1 shock. The deuterated fraction D _(frac) (HCN) is ~ 4 × 10~(-3) and given the associated uncertainties, we did not find significant variations across the bow-shock structure. Contrary to HDCO, whose emission delineates the region of impact between the fast jet and the ambient material, DCN is more widespread and not limited to the impact region. This is consistent with the idea that gas-phase chemistry is playing a major role in the deuteration of HCN in the head of the bow-shock, where HDCO is undetected as it is a product of grain-surface chemistry. The spectra of DCN and H ~(13) CN match the spectral signature of the outflow cavity walls, suggesting that their emission results from shocked gas. The analysis of the time-dependent gas-grain chemical model UCL_CHEM coupled with a parametric C-type shock model shows that the observed deuterated fraction D _(frac) (HCN) is reached during the post-shock phase, when the gas is at T = 80 K, matching the dynamical timescale of the B1 shock, around ~ 1100 yr. Conclusions. Our results indicate that the presence of DCN in L1157-B1 is a combination of gas-phase chemistry that produces the widespread DCN emission, dominating especially in the head of the bow-shock, and sputtering from grain mantles toward the jet impact region, that can be efficient close to the brightest DCN clumps B1a.