Radiation pressure forces of fluffy porous grains

Mie theory and Maxwell-Garnett effective medium theory (MG-Mie) are used to examine a ratio, beta, of the radiation pressure forces to the gravity on porous aggregates with fractal structures, which consist of absorbing or dielectric materials. A comparison of beta by Mg-Mie theory with those computed by the discrete dipole approximation (DDA) shows that when the dust aggregates have rather compact structures and/or the constituent particles are absorbing materials, the values of beta by DDA theory are reproduced reasonably by MG-Mie theory. It is found that, the higher the porosity, the smaller the value of beta becomes near the maximum of beta at a characteristic radius of the aggregate of about 0.1 micron. For very porous aggregates, beta does not depend on size in contrast to a remarkable size dependence of beta for compact spheres.