  The densest infrared dark clouds (IRDCs) may represent the earliest
observable stage of high-mass star formation. These clouds are very cold, hence
they emit mainly at far-infrared and sub-mm wavelengths. For the first time,
Herschel has provided multi-wavelength, spatially resolved observations of
cores within IRDCs, which, when combined with radiative transfer modelling, can
constrain their properties, such as mass, density profile and dust temperature.
We use a 3D, multi-wavelength Monte Carlo radiative transfer code to model in
detail the emission from six cores in three typical IRDCs seen in the Hi-GAL
survey (G030.50+00.95, G031.03+00.26 and G031.03+00.76), and thereby to
determine the properties of these cores and compare them with their low-mass
equivalents. We found masses ranging from 90 to 290 solar masses with
temperatures from 8 to 11K at the centre of each core and 18 to 28K at the
surface. The maximum luminosity of an embedded star within each core was
calculated, and we rule out the possibility of significant high mass star
formation having yet occurred in three of our cores.
