N-BODY SIMULATION OF GRAVITATIONAL INTERACTION BETWEEN PLANETESIMALS AND A PROTOPLANET .2. DYNAMIC FRICTION

We have investigated the dynamical friction of planetesimals by means of three-dimensional N-body simulations to examine whether runaway growth occurs or not. We considered two typical cases: (1) four-component systems with slightly different masses and (2) a protoplanet embedded in a swarm of small planetesimals. In the former case, we find that eccentricity e and inclination i of large mass particles are reduced by the dynamical friction from small mass particles. This result suggests that the runaway growth starts in an early-stage planetesimal swarm which has a continuous mass distribution. In the latter case, e and i of a protoplanet immediately decrease because of the dynamical friction from the planetesimals and settle around the energy equipartition with planetesimals. The decay timescale is well described by a semianalytical theory based on Ida (1990, Icarus 88, 129-145). This result supports further runaway growth of the protoplanet. © 1992.