Excess infrared emission from large interstellar carbon grains

Hydrogenated amorphous carbon (HAC) grains consist of graphitic islands randomly oriented to form a larger particle. These islands (which may be considered to be PAH-like) are poorly connected to each other with respect to energy transfer. An analysis of energy transfer between graphitic islands in HAC shows that absorption of a photon within an individual island does not, in general, heat the entire grain, but heats primarily only the original island and those close to it. Islands can then exhibit temperature fluctuations characteristic of ∼ 10 A particles while remaining part of a larger grain. Large (radius ∼ 0.1 μm)HAC dust will them emit excess IR radiation in the region λ ∼ 20μm, with an effective surface area for IR emission which is about 30 times the geometric surface area of these grains. Such emission may be observed in the IR cirrus. (author)