Grain Formation through Nucleation Process in Astrophysical Environment

General picture of grain formation is preoented based on the nucleation theory. Grain formation process is described by a growth equation of grain radius and an equation of monomer consumption due to the growth of grains. These equations are characterized by two parameters. One depends on the physical conditions of the system and the other reflects the nature of grain materials. An overall feature of the grain formation process is illustrated by the use of an analytic expression of the solutions. After the vapor cools down to the saturated state, a waiting time is necessary until the grain formation begins effectively. Size distribution is relatively sharp in general. The representative size is closely related to the parameter which depends on the physical conditions. Growth by coalescence is not effective until the monomer sticking process is almost completed. The results are applied to the condensation in the primordial solar nebula. It is shown how the picture based on the chemical equilibrium calculations should be modified.