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

摘要

We have analyzed the properties of dust in the high galactic latitudetranslucent cloud Lynds 1780 using ISOPHOT maps at 100 and 200 micrometers andraster scans at 60, 80, 100, 120, 150 and 200 micrometers. In far-infrared(FIR) emission, the cloud has a single core that coincides with the maxima ofvisual extinction and 200um optical depth. At the resolution of 3.0 arcmin, themaximum visual extinction is 4.0 mag. At the cloud core, the minimumtemperature and the maximum 200um optical depth are 14.9+/-0.4 K and2.0+/-0.2x10^{-3}, respectively, at the resolution of 1.5 arcmin. The cloudmass is estimated to be 18M_{SUN}. The FIR observations, combined with IRASobservations, suggest the presence of different, spatially distinct dust grainpopulations 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 maximaon the Eastern side of the cold core, towards the "tail" of thiscometary-shaped cloud. The color ratios indicate an overabundance of PAHs andVSGs in L1780. Our FIR observations combined with the optical extinction dataindicate an increase of the emissivity of the big grain dust component in thecold core, suggesting grain coagulation or some other change in the propertiesof the large grains. Based on our observations, we also address the question,to what extent the 80um emission and even the 100um and the 120um emissioncontain a contribution from the small-grain component.