Composite Grains: Effects of Porosity and Inclusions on the 10μm Silicate Feature

We calculate the absorption efficiency of the composite grains, made up of host silicate spheroids and inclusions of ices/graphites/or voids, in the spectral region $7.0-14.0\mu$m The absorption efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA) as well as using the effective medium approximation & T-Matrix (EMT-Tmatrix) ap proach. We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the $10.0\mu$m feature with the volume fraction of the inclusions and porosity. We then calculate the infrared fluxes for these composite grains and compare the model curves with the average observed IRAS-LRS curve, obtained for several circumstellar dust shells around stars. These results on the composite grains show that the wavelength of the peak absorption shifts and the width of the $10.0\mu$m feature varies with the variation in the volume fraction of the inclusions. The model curves for composite grains with axial ratios not very large (AR$\sim$1.3) and volume fractions of inclusions with f=0.20, and dust temperature of about 250-300$^{\circ}$K, fit the observed emission curves reasonably well.