Abstract
Using the discrete dipole approximation (DDA) we calculate the absorption efficiency of the composite grain, made up of a host silicate spheroid and inclusions of graphite, in the spectral region 5.0-25.0 μm. We study the absorption as a function of the volume fraction of the inclusions. In particular, we study the variation in the 10.0 μm and 18.0 μm emission features with the volume fraction of the inclusions. Using the extinction efficiencies, of the composite grains we calculate the infrared fluxes at several dust temperatures and compare the model curves with the observed infrared emission curves (IRAS-LRS), obtained for circumstellar dust shells around oxygen rich M-type stars.
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