Interstellar extinction by composite grains

Recent studies indicate that interstellar grains are composites of very small individual particles of silicates and carbon glued together into an aggregate. Using the discrete dipole approximation (DDA), we calculate the extinction, scattering and absorption efficiencies for composite grains assumed to be made of a host silicate sphere with embedded graphite inclusions. In particular, we study the extinction as a function of inclusion size and volume fraction. Using the extinction efficiencies of these composite grains, we evaluate the interstellar extinction curve in the wavelength region of $0.55 \mu{\rm m}{-}0.20 \mu{\rm m}$