VARIATIONS IN THE ABUNDANCE OF TRANSIENTLY HEATED PARTICLES WITHIN NEARBY MOLECULAR CLOUDS

IRAS images of molecular clouds in the Chamaeleon, Taurus, and Ursa Major complexes show that the mid-IR emission from transiently heated particles is distributed very differently from the 100 μm emission from large dust grains. The ratio between 12 and 100 μm emission varies by more than one order of magnitude in each complex from ∼5 times to about one-quarter of the average value in the solar neighborhood. Within a complex, color variations are seen on all scales: between clouds a few pc in size and within these clouds on scales as small as a few tenths of a pc. No significant variations of the Iv(100 μm)/Av ratio are observed between clouds of widely different mid- to far-IR color. We show that neither the large amplitude of the color variations nor their morphology can be explained by changes of the excitation by the UV radiation field and conclude that the color variations trace variations in the abundance of transiently heated particles. We propose a scenario which relates the abundance variations to the cycling of interstellar matter between the gas-phase and grain surfaces. In this scenario small particles form in molecular clouds from molecules condensed on grains and photo-processed into organic mantles by UV light.