We develop the mid-infrared extinction ( MIREX ) mapping technique of Butler & Tan ( 2009 , 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 \simeq 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 ( IRDCs ) , 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 _ { \odot } .
We find these objects have a mean radius of R _ { cl } \simeq 0.1 pc , mean \bar { \Sigma } _ { cl } = 0.3 \ > g\ > cm ^ { -2 } and , if fit by a power law density profile \rho _ { cl } \propto 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 , we find a mean core density power law index of k _ { \rho,c } = 1.6 .
We repeat this analysis as a function of radius and derive the best-fitting power law plus uniform clump envelope model for each of the 42 core/clumps .
The cores have typical masses of M _ { c } \sim 100 \ > M _ { \odot } and \bar { \Sigma } _ { c } \sim 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 power laws .
We conclude 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 towards higher densities and steeper density profiles as star formation proceeds .