The influence of magnetic properties on the absorption of porous spheres

To identify the composition of interstellar dust grains by their absorption spectra requires accurate knowledge of their optical properties. The presence of inclusions or voids within grains can alter their refractive indices. Here, we examine the suitability of applying the Maxwell-Garnett mixing theory to highly porous grains. It is found that the effective refractive index obtained for heterogeneous grains depends upon the magnetic permittivities as well as the dielectric constants of its constituent materials. In the case of small grains (radii ∼1μm) consisting of a continuous material inter-dispersed with a number of vacuous inclusions a relatively simple expression for the effective dielectric constant and magnetic permittivity is obtained, from which effective refractive index and absorption efficiency can be calculated. The absorption efficiency for porous, iron grains is calculated over the frequency range 0.5–10 GHz. For such grains, it is found that the absorption efficiency increases as the number of inclusions increases.