| Abstract
| - Context. The isocyanic acid (HNCO) has an extended distribution in the centers of the Milky Way and the spiral galaxy IC342. Based on both the morphology of the emission and the HNCO abundance with respect to H 2, several authors hypothesized that HNCO could be a good tracer of interstellar shocks. Aims. We test this hypothesis by observing a well-known Galactic source whose chemistry is dominated by shocks. Methods. We observed several transitions of HNCO towards L1157-mm and two positions (B1 and B2) in the blue lobe of the molecular outflow. Results. The HNCO line profiles exhibit the same characteristics as other well-known shock tracers such as CH 3OH, H 2CO, SO, or SO 2. The three molecules HNCO, SO 2, and OCS are the only ones detected so far whose emission is more intense in B2 than in B1, making these species valuable probes of chemical differences along the outflow. The HNCO abundance with respect to H 2 is (0.4-1.8)×10 -8 in B1 and (0.3-1)×10 -7 in B2. These abundances are the highest ever measured, and imply an increment with respect to L1157-mm of up to a factor of 83, demonstrating that this molecule is a good shock tracer. Conclusions. we demonstrate that shocks can produce the HNCO abundance measured in galactic nuclei and even higher values. We propose that the gas phase abundance of HNCO is produced by both grain mantle erosion by the shock waves and by neutral-neutral reactions in gas phase involving CN and O 2. The observed anticorrelation between CN and HNCO fluxes supports this scenario. The observed similarities between the HNCO emission and sulfur-bearing molecules may be caused by formation pathways that also involve O 2.
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