THE PHYSICS OF DUST COAGULATION AND THE STRUCTURE OF DUST AGGREGATES IN-SPACE

Even though dust coagulation is a very important dust-processing mecha nism in interstellar space and protoplanetary disks, there are still i mportant parts of the physics involved that are poorly understood. Thi s imposes a serious problem for model calculations of any kind. In thi s paper, we attempt to improve the situation by including the effects of tangential forces on the contact in some detail. These have been st udied in recent papers. We summarize the main results from these paper s and apply them to detailed simulations of the coagulation process an d of collisions between dust aggregates. Our results show the followin g: (1) the growth of aggregates by monomers will normally not involve major restructuring of the aggregates, (2) the classical hit-ann-stick assumption is reasonably valid for this case, (3) collisions of aggre gates with each other or with large grains can lead to significant com paction, and (4) the results can be easily understood in terms of crit ical energies for different restructuring processes. We also derive a short summary that may be used as a recipe for determining the outcome of collisions in coagulation calculations. It is shown that turbulent velocity fields in interstellar clouds are capable of producing consi derably compressed aggregates, while the small aggregates forming earl y on in the solar nebula will not be compacted by collisions. However, compaction provides an important energy sink in collisions of larger aggregates in the solar nebula.