MID-INFRARED EXTINCTION MAPPING OF INFRARED DARK CLOUDS. II. THE STRUCTURE OF MASSIVE STARLESS CORES AND CLUMPS

Abstract

(abridged) We develop the mid-infrared extinction (MIREX) mapping technique\nof Butler & Tan (2009, Paper I), presenting a new method to correct for the\nGalactic foreground emission based on observed saturation in independent cores.\nUsing Spitzer GLIMPSE 8 micron images, this allows us to accurately probe mass\nsurface densities, Sigma, up to ~0.5g/cm^2 with 2" resolution. We then\ncharacterize the structure of 42 massive starless and early-stage IRDC cores\nand their surrounding clumps, measuring Sigma_cl(r) from the core/clump\ncenters. We first assess the properties of the core/clump at a scale where the\ntotal enclosed mass as projected on the sky is M_cl=60Msun. We find these\nobjects have a mean radius of R_cl~0.1pc, mean Sigma_cl=0.3g/cm^2 and, if fit\nby a power law density profile rho_cl ~ r^{-k_{rho,cl}}, a mean value of\nk_{rho,cl}=1.1. If we assume a core is embedded in each clump and subtract the\nsurrounding clump envelope to derive the core properties, we find a mean core\ndensity power law index of k_{rho,c} = 1.6. We repeat this analysis as a\nfunction of radius and derive the best-fitting power law plus uniform clump\nenvelope model for each of the 42 core/clumps. The cores have typical masses of\nM_c~100Msun and mean Sigma_c~0.1g/cm^2, and are embedded in clumps with\ncomparable mass surface densities. We conclude massive starless cores exist and\nare well-described by singular polytropic spheres. Their relatively low values\nof Sigma and the fact that they are IR dark may imply that their fragmentation\nis inhibited by magnetic fields rather than radiative heating. Comparing to\nmassive star-forming cores, there is tentative evidence for an evolution\ntowards higher densities and steeper density profiles as star formation\nproceeds.\n