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The isocyanic acid (HNCO) presents an extended distribution in the centers ofthe Milky Way and the spiral galaxy IC342. Based on the morphology of theemission and the HNCO abundance with respect to H2, several authors made thehypothesis that HNCO could be a good tracer of interstellar shocks. Here wetest this hypothesis by observing a well-known Galactic source where thechemistry is dominated by shocks. We have observed several transitions of HNCOtowards L1157-mm and two positions (B1 and B2) in the blue lobe of themolecular outflow. The HNCO line profiles exhibit the same characteristics ofother well-known shock tracers like CH3OH, H2CO, SO or SO2. HNCO, together withSO2 and OCS, are the only three molecules detected so far whose emission ismuch more intense in B2 than in B1, making these species valuable probes ofchemical differences along the outflow. The HNCO abundance with respect to H2is 0.4-1.8 10^-8 in B1 and 0.3-1 10^-7 in B2. These abundances are the highestever measured, and imply an increment with respect to L1157-mm of a factor upto 83, demonstrating that this molecule is actually a good shock tracer. Ourresults probe that shocks can actually produce the HNCO abundance measured ingalactic nuclei and even higher ones. We propose that the gas phase abundanceof HNCO is due both to grain mantles erosion by the shock waves and byneutral-neutral reactions in gas phase involving CN and O2. The observedanticorrelation of CN and HNCO fluxes supports this scenario. The observedsimilarities of the HNCO emission and the sulfured molecules may arise due toformation pathways involving also O2.

HNCO enhancement by shocks in the L1157 molecular outflow