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 \arcmin , 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 \pm 0.4 K and 2.0 \pm 0.2 \times 10 ^ { -3 } , respectively , at the resolution of 1.5 \arcmin .
The cloud mass is estimated to be 18 M _ { \sun } .
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 .