A SELF-CONSISTENT NUMERICAL TREATMENT OF FRACTAL AGGREGATE DYNAMICS

A self-consistent numerical treatment for modeling fractal aggregate d ynamics is presented. Fractal aggregates play an important role in a n umber of complex astrophysical regimes, including the early solar nebu la and the interstellar medium. Aggregates can be of various forms and sizes, ranging from tiny dust particles to ice chunks in planetary ri ngs and possibly even comets. Many observable properties, such as ligh t scattering and polarization, may depend sensitively on the geometry and motion of such aggregates. Up to now various statistical methods h ave been used to model the growth and interaction of aggregates. The m ethod presented here is unique in that a full treatment of rigid body dynamics-including rotation-is incorporated, allowing individual parti cle and cluster trajectories and orientations to be followed explicitl y. The method involves solving Euler's equations for rigid body motion and introducing a technique for handling oblique collisions between a rbitrarily shaped aggregates. Individual particles may be of any size and can have their own spin. Currently tangential impulses during impa cts are assumed negligible, although equations for the treatment of ta ngential friction are presented. Models for the coagulation and restit ution of aggregates are discussed in detail. Some of the key features required for a fragmentation model, not implemented here, are discusse d briefly. Torque effects arising from self-gravity, tidal fields, or gas drag are not presently considered. Although the discussion focuses mainly on the theory behind the numerical technique, test simulations are presented to compare with an analytic solution of the coagulation equation and to illustrate the important aspects of the method. (C) 1 995 Academic Press, Inc.