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

Abstract

We used FIR and submillimeter continuum data from Herschel and the Atacama Pathfinder EXperiment (APEX) to fit pixel-by-pixel modified Planck SEDs to prestellar and protostellar clumps in the Census of High- and Medium-mass Protostars (CHaMP) ($280^{\circ}<\ell<300^{\circ}$, $-4^{\circ}<b<+2^{\circ}$). We present maps of dust temperature ($T_{\text{d}}$) and H$_2$ column density (\ncol) 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 H$_2$ to chart regional variations in their correspondence, and derive maps of the CO abundance. We find the CO abundance varies by an order of magnitude or more across each region, averaging a few$\times$10$^{-5}$ CO per H$_2$, and that the CO abundance distribution across each clump is correlated in both form and magnitude with environmental conditions, especially $T_{\text{d}}$. This demonstrates that no single CO abundance suffices to convert from $N_{\text{CO}}$ to \ncol, even within a single molecular cloud. We also find that $L/M$ traces $T_{\text{d}}$ almost exclusively, and therefore is not an independent star formation tracer, but minima in $T_{\text{d}}$ almost universally coincide with maxima in \ncol, implying that cooling and density enhancement must be simultaneous steps in prestellar clump evolution. Finally, based on generalized histogram N-PDFs of clump-scale (1-5 pc) and cloud-scale ($\gtrsim10$ pc) samples, we could only obtain dual log-normal and power-law fits to $\sim10\%$ 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.