Composite interstellar grains

Abstract

We present a composite dust grain model that is consistent with the observed interstellar extinction and linear polarization. The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation. The interstellar extinction curve is evaluated in the spectral region 3.40–0.10 μm using the extinction efficiencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve. We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and we find the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than are predicted by the bare silicate/graphite grain models. However, these values are still higher than are implied from the recent interstellar medium values.