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With Planck and Herschel, we now have the spectral coverage and angularresolution required to observe dense and cold molecular clouds. As these cloudsare optically thick at short wavelength but optically thin at long wavelength,it is tricky to conclude anything about dust properties without a propertreatment of the radiative transfer (RT). Our aim is to disentangle the effectsof RT and of dust properties on the variations in the dust emission to provideobservers with keys to analyse the emission arising from dense clouds. We modelcylindrical clouds, illuminated by the ISRF, and carry out full RTcalculations. Dust temperatures are solved using DustEM for amorphous carbonsand silicates, grains coated with carbon mantles, and mixed aggregates ofcarbon and silicate. We allow variations of the grain optical properties withwavelength and temperature. We determine observed colour temperatures, T, andemissivity spectral indices, beta, by fitting the dust emission with modifiedblackbodies, to compare our models with observations. RT effects can neitherexplain the low T nor the increased submm emissivity measured at the centre ofdense clouds, nor the observed beta-T anti-correlation. Adding noise to themodelled data, we show that it is not likely to be the unique explanation forthe beta-T anti-correlation observed in starless clouds. It may be explained byintrinsic variations in the grain optical properties with temperature. As forthe increased submm emissivity and the low T, they have to originate invariations in the grain optical properties, probably caused by their growth toform porous aggregates. We find it difficult to track back the nature of grainsfrom the spectral variations in their emission. Finally, the column density isunderestimated when determined with blackbody fitting because of thediscrepancy between T and the true dust temperature at the cloud centre.

Modelling the dust emission from dense interstellar clouds: disentangling the effects of radiative transfer and dust properties