Free molecular gas drag on fluffy aggregates

Dynamical behaviors of dust particles are governed by gas drag forces in a variety of environments of astrophysical interest, such as protoplanetary disks, upper planetary atmospheres or cometary coma. Monte Carlo (MC) and Quasi Monte Carlo (QMC) simulations have been performed to estimate the drag forces on fluffy aggregates in the free molecular limit, using two different models of molecule-surface interaction. The new computational scheme involves the concurrent evaluation of the rate of molecular collisions and the momentum transfer efficiency. It is found that the gas drag forces on a fluffy aggregate can be approximated by those on a sphere of equivalent area. The deviation from this approximation depends on the model of the molecule-surface interaction and on the size, structure, velocity and especially the orientation of the aggregate. The anisotropy of the drag force is mainly caused by the variation of the projected area when the velocity S, normalized by the mean thermal velocity of the gas, is larger than 1 and by multiple reflections of incident molecules when S < 1.