  The isocyanic acid (HNCO) presents an extended distribution in the centers of
the Milky Way and the spiral galaxy IC342. Based on the morphology of the
emission and the HNCO abundance with respect to H2, several authors made the
hypothesis that HNCO could be a good tracer of interstellar shocks. Here we
test this hypothesis by observing a well-known Galactic source where the
chemistry is dominated by shocks. We have observed several transitions of HNCO
towards L1157-mm and two positions (B1 and B2) in the blue lobe of the
molecular outflow. The HNCO line profiles exhibit the same characteristics of
other well-known shock tracers like CH3OH, H2CO, SO or SO2. HNCO, together with
SO2 and OCS, are the only three molecules detected so far whose emission is
much more intense in B2 than in B1, making these species valuable probes of
chemical differences along the outflow. The HNCO abundance with respect to H2
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 a factor up
to 83, demonstrating that this molecule is actually a good shock tracer. Our
results probe that shocks can actually produce the HNCO abundance measured in
galactic nuclei and even higher ones. We propose that the gas phase abundance
of HNCO is due both to grain mantles erosion by the shock waves and by
neutral-neutral reactions in gas phase involving CN and O2. The observed
anticorrelation of CN and HNCO fluxes supports this scenario. The observed
similarities of the HNCO emission and the sulfured molecules may arise due to
formation pathways involving also O2.
