HNCO: A Molecule Traces Low-velocity Shock
  Using data from MALT90 (Millimetre Astronomy Legacy Team Survey at 90 GHz),
we present molecular line study of a sample of ATLASGAL (APEX Telescope Large
Area Survey of the Galaxy) clumps. Twelve emission lines have been detected in
all. We found that in most sources, emissions of HC$_3$N, HN$^{13}$C, CH$_3$CN,
HNCO and SiO show more compact distributions than those of HCO$^+$, HNC, HCN
and N$_2$H$^+$. By comparing with other molecular lines, we found that the
abundance of HNCO ($\chi$(HNCO)) correlates well with other species such as
HC3N, HNC, C2H, H13CO+ and N2H+. Previous studies indicate the HNCO abundance
could be enhanced by shocks. However, in this study, we found the abundance of
HNCO does not correlate well with that of SiO, which is also a good tracer of
shocks. We suggest this may be because HNCO and SiO trace different parts of
shocks. Our analysis indicates that the velocity of shock traced by HNCO tends
to be lower than that traced by SiO. In the low-velocity shocks traced by HNCO,
the HNCO abundance increases faster than that of SiO. While in the relatively
high-velocity shocks traced by SiO, the SiO abundance increases faster than
that of HNCO. We suggest that in the infrared dark cloud (IRDC) of MSXDC
G331.71+00.59, high-velocity shocks are destroying the molecule of HNCO.
