The correlations deduced by Boulanger et al. from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary HAC model of interstellar dust. In particular, regions of relatively strong 12–μm emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak –μm emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.
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Duley et al. (1993) studied this question.