Extinction by porous silicate and graphite grains

Discrete dipole approximation (DDA) is used to calculate extinction efficiencies of porous silicate and graphite grains. We apply DDA first to the spheroidal solid grains assumed to be made of a large number of small scattering elements (dipoles). Then we systematically reduce the number of dipoles to model the porous grains. The extinction is studied as a function of wavelength, grain size and porosity for both silicate and graphite grains. Finally the extinction efficiencies of the porous silicate and graphite grains are used to evaluate the interstellar extinction curve which is found to fit well with the average observed extinction curve.