Modelling the dust emission from dense interstellar clouds:
  disentangling the effects of radiative transfer and dust properties
  With Planck and Herschel, we now have the spectral coverage and angular
resolution required to observe dense and cold molecular clouds. As these clouds
are optically thick at short wavelength but optically thin at long wavelength,
it is tricky to conclude anything about dust properties without a proper
treatment of the radiative transfer (RT). Our aim is to disentangle the effects
of RT and of dust properties on the variations in the dust emission to provide
observers with keys to analyse the emission arising from dense clouds. We model
cylindrical clouds, illuminated by the ISRF, and carry out full RT
calculations. Dust temperatures are solved using DustEM for amorphous carbons
and silicates, grains coated with carbon mantles, and mixed aggregates of
carbon and silicate. We allow variations of the grain optical properties with
wavelength and temperature. We determine observed colour temperatures, T, and
emissivity spectral indices, beta, by fitting the dust emission with modified
blackbodies, to compare our models with observations. RT effects can neither
explain the low T nor the increased submm emissivity measured at the centre of
dense clouds, nor the observed beta-T anti-correlation. Adding noise to the
modelled data, we show that it is not likely to be the unique explanation for
the beta-T anti-correlation observed in starless clouds. It may be explained by
intrinsic variations in the grain optical properties with temperature. As for
the increased submm emissivity and the low T, they have to originate in
variations in the grain optical properties, probably caused by their growth to
form porous aggregates. We find it difficult to track back the nature of grains
from the spectral variations in their emission. Finally, the column density is
underestimated when determined with blackbody fitting because of the
discrepancy between T and the true dust temperature at the cloud centre.
