Chemical Evolution in High-Mass Star-Forming Regions: Results from the
  MALT90 Survey
  The chemical changes of high-mass star-forming regions provide a potential
method for classifying their evolutionary stages and, ultimately, ages. In this
study, we search for correlations between molecular abundances and the
evolutionary stages of dense molecular clumps associated with high-mass star
formation. We use the molecular line maps from Year 1 of the Millimetre
Astronomy Legacy Team 90 GHz (MALT90) Survey. The survey mapped several hundred
individual star-forming clumps chosen from the ATLASGAL survey to span the
complete range of evolution, from prestellar to protostellar to H II regions.
The evolutionary stage of each clump is classified using the Spitzer
GLIMPSE/MIPSGAL mid-IR surveys. Where possible, we determine the dust
temperatures and H2 column densities for each clump from Herschel Hi-GAL
continuum data. From MALT90 data, we measure the integrated intensities of the
N2H+, HCO+, HCN and HNC (1-0) lines, and derive the column densities and
abundances of N2H+ and HCO+. The Herschel dust temperatures increase as a
function of the IR-based Spitzer evolutionary classification scheme, with the
youngest clumps being the coldest, which gives confidence that this
classification method provides a reliable way to assign evolutionary stages to
clumps. Both N2H+ and HCO+ abundances increase as a function of evolutionary
stage, whereas the N2H+ (1-0) to HCO+ (1-0) integrated intensity ratios show no
discernable trend. The HCN (1-0) to HNC(1-0) integrated intensity ratios show
marginal evidence of an increase as the clumps evolve.
