Properties of dust in the high-latitude translucent cloud L1780 -: I.: Spatially distinct dust populations and increased dust emissivity from ISO observations

We have analyzed the properties of dust in the high galactic latitude translucent cloud Lynds 1780 using ISOPHOT maps at 100 mu m and 200 mu m and raster scans at 60 mu m, 80 mu m, 100 mu m, 120 mu m, 150 mu m and 200 mu m. In far-infrared (FIR) emission, the cloud has a single core that coincides with the maxima of visual extinction and 200 mu m optical depth. At the resolution of 3.0', the maximum visual extinction is 4.0 mag. At the cloud core, the minimum temperature and the maximum 200 mu m optical depth are 14.9 +/- 0.4 K and 2.0 +/- 0.2 x 10(-3), respectively, at the resolution of 1.5'. The cloud mass is estimated to be 18 M-circle dot. The FIR observations, combined with IRAS observations, suggest the presence of different, spatially distinct dust grain populations in the cloud: the FIR core region is the realm of the classical large grains, whereas the very small grains and the PAHs have separate maxima on the Eastern side of the cold core, towards the tail of this cometary-shaped cloud. The color ratios indicate an overabundance of PAHs and VSGs in L1780. Our FIR observations combined with the optical extinction data indicate an increase of the emissivity of the big grain dust component in the cold core, suggesting grain coagulation or some other change in the properties of the large grains. Based on our observations, we also address the question, to what extent the 80 mu m emission and even the 100 mu m and the 120 mu m emission contain a contribution from the small-grain component.