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 HN ¹³C, SO and ³⁴SO, CCH, SO₂, and CH ₃OH-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 N ₂H⁺ integrated intensity does not correlate well with dust continuum flux. We calculate the effective excitation density, n_eff, the density required to excite a 1 K line in T_kin = 20 K gas for each molecular tracer. The intensity of molecular tracers with larger effective excitation densities (n_eff ≥ 10⁵cm^-3) appears to correlate more strongly with the submillimeter dust continuum intensity. The median sizes of the clumps are anti-correlated with the n _eff of the tracers (which span more than three orders of magnitude). Virial mass is not correlated with n_eff, 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 n_eff indicating the importance of understanding the excitation conditions of the molecular tracer when deriving the average properties of an ensemble of cores.