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Properties of dust in the high-latitude translucent cloud L1780 - II. 3D radiative transfer modelling

Context. Lynds 1780 is a high-latitude cloud where, based on 2MASS, the maximum visual extinction is A max V = 4 mag at a resolution of 3'. In LDN1780, increased far-infrared (FIR) emissivity of dust grains has been observed, and the infrared emission is found to peak at different locations at different wavelengths. Aims. By modelling the FIR observations, we try to quantify spatial variations of dust properties and to determine to what extent the observations could be affected by the asymmetry of the heating radiation field. Methods. We have constructed a three-dimensional cloud model and, with the help of radiative transfer calculations, compare its predictions with the FIR surface brightness measurements of LDN1780 performed with the ISO satellite. The effects of anisotropic radiation, its attenuation in a diffuse extinction layer around the cloud, and variations in the dust properties are investigated. Results. Asymmetry of the radiation field is found to have only a small effect on the morphology of mid- and far-infrared surface brightness. The general agreement between observations and the model predictions is improved by assuming the presence of a low extinction external layer with A V ~ 0.25 mag. However, to explain the changes in the relative intensity of mid- and far-infrared bands, one has to assume strong variations in the relative abundance of small and large grain components and, at the very centre of the cloud, enhanced emissivity of large grains. Conclusions. The separate emission maxima at different wavelengths in LDN1780 result from real variations in spatial distributions of dust components. Modifications to standard dust models, including a 30% increase in the FIR emissivity, are needed to explain the far-infrared observations towards the centre of LDN1780. The relative abundances of dust components are found to be very sensitive to the strength of the external radiation field.