Abstract
A composite dust grain model which simultaneously explains the observed interstellar extinction, polarization, IR emission and the abundance constraints, is required. We present a composite grain model, which is made up of a host silicate oblate spheroid and graphite inclusions. The interstellar extinction curve is evaluated in the spectral region 3.4-0.1$\mu m$ using the extinction efficiencies of the composite spheroidal grains for three axial ratios. Extinction curves are computed using the discrete dipole approximation (DDA). The model curves are subsequently compared with the average observed interstellar extinction curve and with an extinction curve derived from the IUE catalogue data.
Highlights
It is highly unlikely that interstellar grains are spherical in shape or that they are homogeneous in composition and structure
Our main conclusions from this study are: (1) The extinction curves for the composite spheroidal grains show a shift in the central wavelength of the extinction peak as well as a variation in the width of the peak with a variation in the volume fraction of the graphite inclusions
These results clearly indicate that the shape, structure and inhomogeneity in the grains play an important role in producing the extinction
Summary
It is highly unlikely that interstellar grains are spherical in shape or that they are homogeneous in composition and structure. We calculate the extinction efficiencies for composite oblate spheroidal grains, made up of the host silicate spheroid with embedded inclusions of graphite, in the wavelength region 3.4–0.10 μm.
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