2013MNRAS.432.2231B

We report new near-infrared and mm-wave observational data on a selection of massive Galactic molecular clumps (part of the Census of High- and Medium-mass Protostars 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 Brγ 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 Brγ 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 Brγ nebulosity, while bright N2H+ emission tends to avoid areas that are bright in Brγ. 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 Young Stellar Objects 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±Brγ relationship is a useful diagnostic of a molecular clump's progress in forming massive stars.