2012ApJ...754....5B

We develop the mid-infrared extinction (MIREX) mapping technique of Butler & Tan (Paper I), presenting a new method to correct for the Galactic foreground emission based on observed saturation in independent cores. Using Spitzer GLIMPSE 8 µm images, this allows us to accurately probe mass surface densities, Σ, up to ≃ 0.5 g/cm2 with 2'' resolution and mitigate one of the main sources of uncertainty associated with Galactic MIREX mapping. We then characterize the structure of 42 massive starless and early-stage cores and their surrounding clumps, selected from 10 infrared dark clouds, measuring Σ_cl(r) from the core/clump centers. We first assess the properties of the core/clump at a scale where the total enclosed mass as projected on the sky is M_cl= 60 M_☉. We find that these objects have a mean radius of R_cl≃ 0.1 pc, mean _cl = 0.3 g/cm2 and, if fitted by a power-law (PL) density profile ρ_cl ∝r^–kρ,cl_^, a mean value of k_ρ, cl_= 1.1. If we assume a core is embedded in each clump and subtract the surrounding clump envelope to derive the core properties, then we find a mean core density PL index of k_ρ, c_= 1.6. We repeat this analysis as a function of radius and derive the best-fitting PL plus uniform clump envelope model for each of the 42 core/clumps. The cores have typical masses of M_c∼ 100 M_☉ and _c ∼ 0.1 g/cm2, and are embedded in clumps with comparable mass surface densities. We also consider Bonnor-Ebert density models, but these do not fit the observed Σ profiles as well as PLs. We conclude that massive starless cores exist and are well described by singular polytropic spheres. Their relatively low values of Σ and the fact that they are IR dark may imply that their fragmentation is inhibited by magnetic fields rather than radiative heating. Comparing to massive star-forming cores and clumps, there is tentative evidence for an evolution toward higher densities and steeper density profiles as star formation proceeds.