The Spheroidal Analog Method for Modeling Irregular Porous Aggregates

It is shown that the optical properties of an irregular porous grain with effective radius $a_{\rm eff}\lesssim 3\lambda$ (where $\lambda$ is the wavelength) can be well approximated by a ``spheroidal analog'': a spheroid with appropriate axial ratio and size, with a dielectric function obtained from an effective medium theory. Prescriptions for specifying the axial ratio and porosity of the spheroidal analog, based on simple geometric properties, are given. The accuracy of the spheroidal analogue method is studied for irregular grains with a range of structures and porosities. Different effective medium theories are compared; Bruggeman's theory is found to give the best results. The accuracy of the spheroidal analog method justifies the use of spheroids for modeling absorption, scattering and polarization by interstellar, circumstellar, or interplanetary dust.