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 mu m images, this allows us to accurately probe mass surface densities, Sigma, up to similar or equal to 0.5g 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 Sigma(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-circle dot. We find that these objects have a mean radius of R-cl similar or equal to 0.1 pc, mean (Sigma) over bar (cl) = 0.3 g cm(-2) and, if fitted by a power-law (PL) density profile rho(cl) proportional to r(-k rho,cl), a mean value of k(rho,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(rho,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 similar to 100 M-circle dot and (Sigma) over bar (c) similar to 0.1 g cm(-2), and are embedded in clumps with comparable mass surface densities. We also consider Bonnor-Ebert density models, but these do not fit the observed Sigma 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 Sigma 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.