The Role of Outflows and C Shocks in the Strong Deuteration of L1689N

Abstract

Recent detections of doubly deuterated ammonia and formaldehyde in L134N and L1689N have renewed interest in deuterium fractionation in cold, dense molecular gas. Chemical models show that the level of fractionation depends critically on the physical conditions, namely, the temperature and density of the molecular gas. Therefore, detailed studies of cold, dense molecular cores are required to constrain the existing chemical networks. We have mapped the core of the remarkable star-forming molecular cloud L1689N in a number of molecular tracers, including singly deuterated species, in order to obtain detailed information on the morphology and kinematics of the molecular gas, as well as the physical conditions in the dense core and the shocked regions associated with the molecular outflows emanating from the embedded far-infrared source IRAS 16293-2422. Our data suggest the presence of two regions of interaction between the molecular outflows and the dense ambient cloud, one of them associated with the SiO peak E1, the other one near the peak of deuterated and doubly deuterated molecular species ~90'' east of the IRAS source. The observed intensities of the CO (3-2), (4-3), and (6-5) emission at these two locations are consistent with those predicted by C shock models with velocities of 15-25 and 8-10 km s-1, respectively. The relatively low shock velocity derived for the deuterium peak explains why no SiO emission has been detected at this location. Deuterated molecular species, such as DCO+ and DCN, in the core of L1689N are kinematically and morphologically distinct from other molecular tracers of the dense gas, such as HCO+, H13CO+, and HCN. The peak of deuterated molecules is also displaced from the extended millimeter dust continuum source located in this region. We use formaldehyde observations to derive the excitation conditions in the dense gas and, subsequently, the D/H ratios in HCO+ and HCN at several locations via large velocity gradient modeling. These ratios are typically ~1% toward the locations of the IRAS source and the SiO peak E1, with much higher values, ~10%, derived toward the deuterium peak. These values fall within the range of predictions of gas-phase chemical models for dense and cold gas, provided that significant accretion of H2O, CO, and other molecular species on the dust grains has occurred. However, some discrepancies between the observations and model predictions remain. For example, contrary to gas-phase model predictions, we derive a higher deuterium fractionation in HCN compared to HCO+.