EXTINCTION PROPERTIES OF DUST GRAINS - A NEW COMPUTATIONAL TECHNIQUE

The Strong Permittivity Fluctuation Theory (SPFT) is a statistical app roach to describe the optical. properties of randomly oriented inhomog eneous dust grains. In contrast to most other methods that must calcul ate the extinction properties of individual particles and then sum the results to obtain ensemble averages, the SPFT starts with ensemble-av eraged quantities from the very beginning. Therefore, the computation of the extinction has to be performed only once for a given ensemble. This makes this method especially suitable for astrophysical applicati ons in which grain alignment is not required. We use the SPFT in biloc al approximation (BA) to calculate the orientationally and positionall y averaged extinction cross section of dust agglomerates. In the BA, t he average extinction cross section of a statistical ensemble of grain s depends only on the complex refractive index of the constituent part icles, and on the value and spatial correlation of the local density ( filling factor) of the material. The BA is tested for the case of aggl omerates of spherical grains by comparing its predictions to those of the discrete dipole approximation (DDA) and the generalized Mie theory (GMT), which embodies an exact solution of Maxwell's equations. We fi nd that the BA works well for all size parameters tested (up to 0.6, t aking the radius of the individual constituent spheres). We find a ver y good agreement between the BA, DDA, and GMT for a moderate refractiv e index (n(ref)=1.7+0.7i, typical of soot or ''dirty'' silicate cosmic dust analogs in the visual). For a somewhat larger refractive index ( n(ref)=2.5+1.5i, typical of graphite in the UV), the BA is of comparab le accuracy as the DDA. For the more extreme case of a quasimetallic r efractive index (n(ref)=3.0+4.0i), the BA seems to be an inadequate ap proach. For this case, all methods are pushed to their limit.