Spatial Variation of the Chemical Properties of Massive Star-forming
  Clumps
  We selected 90 massive star-forming clumps with strong N2H+, HCO+, HCN, and
HNC emission from the Millimetre Astronomy Legacy Team 90 GHz survey. We
obtained Herschel data for all 90 sources and NRAO VLA Sky Survey data for 51
of them. We convolved and regridded all images to the same resolution and pixel
size and derived the temperature, H2 column density, molecules' abundances and
abundance, and ratios of each pixel. Our analysis yields three main
conclusions. First, the abundances of N2H+, HCO+, HCN, and HNC increase when
the column density decreases and the temperature increases, with spatial
variations in their abundances dominated by changes in the H2 column density.
Second, the abundance ratios between N2H+, HCO+, HCN, and HNC also display
systemic variations as a function of the column density due to the chemical
properties of these molecules. Third, the sources associated with the 20 cm
continuum emission can be classified into four types based on the behavior of
the abundances of the four molecules considered here as a function of this
emission. The variations of the first three types could also be attributed to
the variation of the H2 column density.
