Radiation pressure cross-sections of fluffy interstellar grains

We computed, through the transition matrix method, the radiation pressure cross-sections of cosmic dust grains modelled as aggregates (clusters) of spheres of appropriate geometry. The calculation is performed without resorting to any approximation and with a computational effort that is noticeably lighter than the one required by other methods. Our results show that radiation pressure cross-sections decrease with increasing particle fluffiness in the near- ultraviolet and visible range of the spectrum. This is due to the decrease of the corresponding strength of the multiple scattering processes that couple the aggregated spheres to each other. As a result, the inertial response to radiation forces of highly porous aggregates tends to become similar to that of the constituent particles.