INFRARED-EMISSION FROM ISOLATED DUST CLOUDS IN THE PRESENCE OF VERY SMALL DUST GRAINS

We have developed a continuum radiation transport code which incorporates the transient heating of small dust grains. Using this computer code, we have constructed models to study the effects of the temperature fluctuations due to small grains on the infrared spectrum and surface brightness of externally heated interstellar dust clouds. From comparison of the model results with IRAS observations, we conclude that the observed emission in the 12- and 25-μm bands is most likely produced by small grains with radius of ∼10 Å, while the 60- and 100-μm emission comes primarily from large grains heated under the equilibrium conditions. Our models, based on the assumption of a constant fractional abundance of large and small grains throughout a cloud, predict infrared limb brightning only for models with a visual optical depth {greater-than or approximate} 20. The fact that relatively strong limb brightening has been observed for more diffused globules indicates that the small grain abundance is likely to be increased in outer parts of these objects. © 1991.