Mid-Infrared Extinction Mapping of Infrared Dark Clouds. Ii. The Structure of Massive Starless Cores and Clumps

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 cm −2 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 and, if fitted by a power-law (PL) density profile , 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 , 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.