Millimetre-Wave and Near-Infrared Signposts of Massive Molecular Clump
  Evolution and Star Cluster Formation
  We report new near-infrared and mm-wave observational data on a selection of
massive Galactic molecular clumps (part of the CHaMP sample) and their
associated young star clusters. The clumps show, for the first time in a "dense
gas tracer", a significant correlation between HCO+ line emission from cold
molecular gas and Br{\gamma} line emission of associated nebulae. This
correlation arises in the HCO+ line's brightness, not its linewidth. In
contrast, the correlation between the N2H+ line emission and Br{\gamma} is weak
or absent. The HCO+/N2H+ line ratio also varies widely from clump to clump:
bright HCO+ emission tends to be more closely associated with Br{\gamma}
nebulosity, while bright N2H+ emission tends to avoid areas that are bright in
Br{\gamma}. Both molecular species show correlations of weak significance with
infrared H2 v=1-0 and v=2-1 line emission, in or near the clumps. The H2
emission line ratio is consistent with fluorescent excitation in most of the
clumps, although thermal excitation is seen in a few clumps. We interpret these
trends as evidence for evolution in the gas conditions due to the effects of
ongoing star formation in the clumps, in particular, the importance of UV
radiation from massive YSOs as the driving agent that heats the molecular gas
and alters its chemistry. This suggests that some traditional dense gas tracers
of molecular clouds do not sample a homogeneous population of clumps, i.e.,
that the HCO+ brightness in particular is directly related to the heating and
disruption of cold gas by massive young stars, whereas the N2H+ better samples
gas not yet affected by this process. We therefore suggest that the
HCO+-N2H+-Br{\gamma} relationship is a useful diagnostic of a molecular clump's
progress in forming massive stars.
