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The gravitational instability of a dust layer is one of the scenarios forplanetesimal formation. If the density of a dust layer becomes sufficientlyhigh as a result of the sedimentation of dust grains toward the midplane of aprotoplanetary disk, the layer becomes gravitationally unstable andspontaneously fragments into planetesimals. Using a shearing box method, weperformed local $N$-body simulations of gravitational instability of a dustlayer and subsequent coagulation without gas and investigated the basicformation process of planetesimals. In this paper, we adopted the accretionmodel as a collision model. A gravitationally bound pair of particles isreplaced by a single particle with the total mass of the pair. This accretionmodel enables us to perform long-term and large-scale calculations. Weconfirmed that the formation process of planetesimals is the same as that inthe previous paper with the rubble pile models. The formation process isdivided into three stages: the formation of non-axisymmetric structures, thecreation of planetesimal seeds, and their collisional growth. We investigatedthe dependence of the planetesimal mass on the simulation domain size. We foundthat the mean mass of planetesimals formed in simulations is proportional to$L_y^{3/2}$, where $L_y$ is the size of the computational domain in thedirection of rotation. However, the mean mass of planetesimals is independentof $L_x$, where $L_x$ is the size of the computational domain in the radialdirection if $L_x$ is sufficiently large. We presented the estimation formulaof the planetesimal mass taking into account the simulation domain size.

N-Body Simulation of Planetesimal Formation through Gravitational Instability and Coagulation. II. Accretion Model