THE PHYSICAL PROPERTIES OF HIGH-MASS STAR-FORMING CLUMPS: A SYSTEMATIC COMPARISON OF MOLECULAR TRACERS

We present observations of HCO+ and H13CO+, N2H+, HCS+, HNC and HN13C, SO and 34SO, CCH, SO2, and CH3OH-E toward a sample of 27 high-mass clumps coincident with water maser emission. All transitions are observed with or convolved to nearly identical resolution (30''), allowing for inter-comparison of the clump properties derived from the mapped transitions. We find that N2H+ emission is spatially differentiated compared with the dust and the other molecules toward a few very luminous cores (10 of 27) and the N2H+ integrated intensity does not correlate well with dust continuum flux. We calculate the effective excitation density, neff, the density required to excite a 1 K line in Tkin = 20 K gas for each molecular tracer. The intensity of molecular tracers with larger effective excitation densities (neff ≥ 105 cm-3) appears to correlate more strongly with the submillimeter dust continuum intensity. The median sizes of the clumps are anti-correlated with the neff of the tracers (which span more than three orders of magnitude). Virial mass is not correlated with neff, especially where the lines are optically thick as the linewidths may be broadened significantly by non-virial motions. The median mass surface density and median volume density of the clumps are correlated with neff indicating the importance of understanding the excitation conditions of the molecular tracer when deriving the average properties of an ensemble of cores.