On the diagnostic power of FIR/sub-mm SED fitting in massive galactic molecular clumps

Abstract

We used far-infrared (FIR) and sub-millimetre continuum data from Herschel and the Atacama Pathfinder EXperiment (APEX) to fit pixel-by-pixel modified Planck spectral energy distributions to prestellar and protostellar clumps in the Census of High- and Medium-mass Protostars (CHaMP) (280° < ℓ < 300°, -4° < b < +2°). We present maps of dust temperature (T_d) and H₂ column density (N_{H_2}) for molecular clumps in the Carina Nebula complex (Regions 9 through 11), and surrounding RCW 64 (Region 26). We compare the column densities of CO and H₂ to chart regional variations in their correspondence, and derive maps of the CO abundance. We find that the CO abundance varies by an order of magnitude or more across each region, averaging a few × 10^-5 CO per H₂, and that the CO abundance distribution across each clump is correlated in both form and magnitude with environmental conditions, especially T_d. This demonstrates that no single CO abundance suffices to convert from N_CO to N_{H_2}, even within a single molecular cloud. We also find that L/M traces T_d almost exclusively, and therefore is not an independent star formation tracer, but minima in T_d almost universally coincide with maxima in N_{H_2}, implying that cooling and density enhancement must be simultaneous steps in prestellar clump evolution. Finally, based on generalized histogram column density probability distribution functions (N-PDFs) of clump-scale (1-5 pc) and cloud-scale (≳10 pc) samples, we could only obtain dual lognormal and power-law fits to {∼ }10 per cent of the clumps. The physical parameters derived from these fits approach theoretical expectations, but have largely unknown uncertainties, so we advise treating the results of N-PDF fitting with caution.