Detection of OD towards the low-mass protostar IRAS 16293-2422.

Context. Although water is an essential and widespread molecule in star-forming regions, its chemical formation pathways are still not very well constrained, observing the level of deuterium fractionation of OH, a radical involved in the water chemical network, is a promising way to infer its chemical origin. Aims. We aim at understanding the formation mechanisms of water by investigating the origin of its deuterium fractionation. This can be achieved by observing the abundance of OD towards the low-mann protostar IRAS 16293-2422, where the FIDO distribution is already known. Methods. Using the GREAT receiver on board SOFIA, we observed the ground-state OD transition at 1391.5 GHz towards the low- mans protostar [RAS 16293-2422. We also present the detection of the HDO 1₁₁-0∞ line using the APEX telescope. We compare the OD/FIDO abundance ratio inferred from these observations with the predictions of chemical models. Results. The OD line in detected in absorption towards the source continuum. Thin in the first detection of OD outside the solar system. The SOFIA observation, coupled to the observation of the HDO 1₁₁-0∞ line, provides an estimate of the abundance ratio OD/HDO ∼ 17-90 in the gas where the absorption taken place. Thin value in fairly high compared with model predictions. Thin may be reconciled if reprocessing in the gas by means of the dissociative recombination of H₂DO⁺ further fractionates OH with respect to water. Conclusions. The present observation demonstrates the capability of the SOFIA/GREAT instrument to detect the ground transition of 00 towards star-forming regions in a frequency range that wan not accessible before. Dissociative recombination of H₂DO⁺ may play an important role in netting a high OD abundance. Measuring the branching ration of this reaction in the laboratory will be of great value for chemical models. [ABSTRACT FROM AUTHOR]