Infrared emission from the composite grains: effects of inclusions and porosities on the 10 and 18 <i>μ</i>m features

Abstract

Aims. In this paper we study the effects of inclusions and porosities on the emission properties of silicate grains and compare the model curves with the observed infrared emission from circumstellar dust. Methods. We calculated the absorption efficiency of the composite grain, made up of a host silicate oblate spheroid and inclusions of ice/graphite/or voids, in the spectral region 5.0-25.0 m. The absorption efficiencies of the composite spheroidal oblate grains for three axial ratios were computed using the discrete dipole approximation (DDA). We studied the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we studied the variation in the 10 m and 18 m emission features with the volume fraction of the inclusions and porosities. We then calculated the infrared fluxes for these composite grains at several dust temperatures (T = 200-350 K) and compared the model curves with the average observed IRAS-LRS curve, obtained for circumstellar dust shells around oxygen rich M-type stars. The model curves were also compared with two other individual stars.