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 7.0-14.0μm. 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 and T-matrix (EMT-T-matrix) approach. We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10.0μm 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 10.0μm 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∼1.3) and volume fractions of inclusions with f=0.20, and dust temperature of about 250-300 K, fit the observed emission curves reasonably well.