  We demonstrate the use of the 3D Monte Carlo radiative transfer code PHAETHON
to model infrared-dark clouds (IRDCs) that are externally illuminated by the
interstellar radiation field (ISRF). These clouds are believed to be the
earliest observed phase of high-mass star formation, and may be the high-mass
equivalent of lower-mass prestellar cores. We model three different cases as
examples of the use of the code, in which we vary the mass, density, radius,
morphology and internal velocity field of the IRDC. We show the predicted
output of the models at different wavelengths chosen to match the observing
wavebands of Herschel and Spitzer. For the wavebands of the long- wavelength
SPIRE photometer on Herschel, we also pass the model output through the SPIRE
simulator to generate output images that are as close as possible to the ones
that would be seen using SPIRE. We then analyse the images as if they were real
observations, and compare the results of this analysis with the results of the
radiative transfer models. We find that detailed radiative transfer modelling
is necessary to accurately determine the physical parameters of IRDCs (e.g.
dust temperature, density profile). This method is applied to study
G29.55+00.18, an IRDC observed by the Herschel Infrared Galactic Plane survey
(Hi-GAL), and in the future it will be used to model a larger sample of IRDCs
from the same survey.
