Millimetre-Wave and Near-Infrared Signposts of Massive Molecular Clump Evolution and Star Cluster Formation

Abstract

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 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 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+-Brγ relationship is a useful diagnostic of a molecular clump's progress in forming massive stars.

28 pages, 38 figures with 63 panels, 3 tables; accepted by MNRAS; high-resolution version (including full appendix) available from "Papers" page at CHaMP website, http://www.astro.ufl.edu/champ

References in corpus (4)

• The Relationship Between Molecular Gas Tracers and Kennicutt-Schmidt Laws
• Molecular Star Formation Rate Indicators in Galaxies
• Multi-wavelength observations of Southern Hot Molecular Cores traced by methanol masers - I. Ammonia and 24 GHz Continuum Data
• Chemical differentiation in regions of high-mass star formation I. CS, dust and N2H^+ in southern sources

Cited by in corpus (13)

• MALT90: The Millimetre Astronomy Legacy Team 90 GHz Survey
• Chemical evolution in the early phases of massive star formation. I
• The Bolocam Galactic Plane Survey. X. A Complete Spectroscopic Catalog of Dense Molecular Gas Observed toward 1.1 mm Dust Continuum Sources with 7.5 <= l <= 194 degrees
• The Galactic Census of High- and Medium-mass Protostars. IV. Molecular Clump Radiative Transfer, Mass Distributions, Kinematics, and Dynamical Evolution
• The Galactic Census of High- and Medium-mass Protostars. III ${}^{12}$CO Maps and Physical Properties of Dense Clump Envelopes and their Embedding GMCs
• HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
• On the Diagnostic Power of FIR/Sub-mm SED Fitting in Massive Galactic Molecular Clumps
• Census of High- and Medium-mass Protostars V. CO Abundance and the Galactic $X_{\text{CO}}$ Factor
• SOFIA and ALMA Investigate Magnetic Fields and Gas Structures in Massive Star Formation: The Case of the Masquerading Monster in BYF 73
• HCN hyperfine ratio analysis of massive molecular clumps
• Gemini, SOFIA, and ATCA Reveal Very Young, Massive Protostars in the Collapsing Molecular Cloud BYF 73
• Physical Properties and Kinematics of Dense Cores Associated with Regions of Massive Star Formation from the Southern Sky
• Dust Temperature Profiles in Dense Cores Associated With Massive Star-Forming Regions