Distribution of HNCO 5$_{05}-4_{04}$ in Massive Star-forming Regions
  The goal of this paper is to study the spatial distribution of HNCO in
massive star-forming regions, and investigate its spatial association with
infrared sources, as well as physical conditions in region of HNCO emission. We
have mapped nine massive star-forming regions in HNCO 5$_{05}-4_{04}$ with the
Purple Mountain Observatory 13.7m telescope. The C18O maps of these sources
were obtained simultaneously. The HNCO emission shows compact distribution,
with emission peak centred on water masers. Nearly all the HNCO clumps show
signs of embedded mid-infrared or far-infrared sources. The FWHM sizes of HNCO
clumps are significantly smaller than C18O clumps but rather similar to HC3N
clumps. We also found good correlation between the integrated intensities,
linewidths and LSR velocities of HNCO and HC3N emission, implying similar
excitation mechanism of these two species. As such, collisional excitation is
likely to be the dominant excitation mechanism for HNCO 5$_{05}-4_{04}$
emission in galactic massive star-forming regions.
