IRAS DETECTION OF VERY COLD DUST IN THE LYNDS-134 CLOUD COMPLEX

We have analyzed IRAS maps at 60 and 100-mu-m of the complex of dark clouds containing L134, L183 and L1780. Regions were found that are bright at 100-mu-m but not detected at 60-mu-m. These regions exhibit high optical opacities and are associated with the dark clouds. The 60-mu-m surface brightness is observed to decline in a narrow transition layer surrounding the cloud centers. In the outer environs the ratio I60/I100 has a nearly constant value of 0.2. The absence of 60-mu-m emission yields an upper limit of 15 K for the temperature of the dust, assuming lambda-1 dust emissivity. We have mapped the 60-mu-m deficient regions using the quantity DELTA-I100 = I100 - I60/Kelvin, where Kelvin is the ratio I60/I100 in the outer diffuse regions. We have found a linear correlation between DELTA-I100 and both (CO)-C-13 column density and blue extinction. The analysis suggests that the diffuse component of the cloud has been subtracted when constructing DELTA-I100, and that DELTA-I100 is tightly correlated with molecular material with densities in excess of n(H-2) = 10(3). We have derived an upper limit for the mass absorption coefficient of dust at 100-mu-m of 4.8 g cm-2. This limit is consistent with earlier determinations. The sudden drop in the 60-mu-m emission can only be explained when assuming a separate grain component at 60-mu-m that undergoes a change in properties in a narrow transition region. We infer that condensation of a thin mantle on the grains in the transition layer can explain the observed infrared properties.