Oligarchic growth of protoplanets in planetesimal rings

Abstract

In the standard planet formation scenario, planetesimals are assumed to form throughout the protoplanetary disk and to be smoothly distributed in the radial direction except for the snowline. Planetesimal growth has been investigated using this assumption, and the oligarchic growth model is widely accepted. However, recent simulations of gas and dust evolution have shown that planetesimals form only in radially limited locations -- such as at gas pressure bumps and snowlines -- and are concentrated in ring-like regions. When planetesimals are distributed in a ring-like region, scattered ones leak from the ring edge, resulting in planetesimal diffusion. To investigate protoplanet growth in expanding planetesimal rings, we perform a series of $\textit{ N}$-body simulations. In all the simulations, protoplanet growth is well explained by oligarchic growth, while the ring width expands due to planetesimal scattering by the protoplanets. Massive protoplanets tend to form near the ring center, and protoplanets that form far from the ring center are less massive than those in the center. The scaled orbital separations depend on neither the initial ring width nor on the total mass, and they are consistent with estimates based on the oligarchic growth model and the diffused planetesimal distribution. The width of the expanded planetesimal ring does not depend on its initial width, but it does depend on its total mass. The maximum mass of protoplanets depends strongly on the total ring mass and weakly on its initial width.