Abstract
Dust grains in central regions of the Orion nebula are solid fossils of molecular cloud material dispersed during episodes of OB star formation. Here, a grain model in which the nebular dust has a silicate and carbon composition and a bimodal size spectrum characterized by a division into large and small particles is considered. The large grains consist of porous aggregates of silicate subgrains, whereas small grains consist of silicate and amorphous carbon particles and a smaller population of amorphous carbon particles graphitized by OB stellar radiation. It turns out that large carbon grains are absent in the size spectrum because they are subjected to chemical attacks by gas-phase H and O atoms.
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