Coagulation of charged dust in protoplanetary disks
Dust evolution is an important part of advanced chemical models of protoplanetary disks. Most of dust evolution models, both standalone and incorporated into thermo-chemical codes, typically considers neutral or singly charged grains. Grain charging in protoplanetary disks and its impact on the dust growth lacks currently a detailed scientific scrutiny. In this work we consider two of the primary mechanisms of the dust charging – photoelectric effect and collision with electrons and ions. To study the impact of the grain charge on the dust coagulation we solve the Smoluchowski equation locally in the (r,z)-plane for typical disk parameters. The collisional charging, which dominates in the disk midplane, leads to electrostatic repulsion between negatively charged grains. This poses a known problem of the electrostatic barrier against dust growth, that significantly hampers dust growth. In the disk atmosphere, where photoelectric charging operates and grains are positively charged, dust retains its initial size distribution both due to low number density and electrostatic repulsion. The most interesting case is at intermediate heights near the zero charge surface, where collisional and photoelectric charging mechanisms are comparable. The presence of both negatively and positively charged grains at the intermediate heights leads to a significant increase in dust coagulation rates, which competes even with coagulation rates in the disk midplane. Such a vertically non-uniform dust evolution affects total available grain surface and UV field distribution, which are of great importance for disk chemistry.