OLIGARCHIC GROWTH OF PROTOPLANETS

We investigate the growth and the orbital evolution of protoplanets em bedded in a swarm of planetesimals using three-dimensional N-body simu lations. We find that among protoplanets, larger ones grow more slowly than smaller ones, while the growth of protoplanets is still faster t han that of planetesimals. As a result, in the stage after rapid runaw ay growth, protoplanets with the same order mass grow oligarchically, while most planetesimals remain small, While the protoplanets grow, or bital repulsion keeps their orbital separations wider than about 5 Hil l radius of the protoplanets. The typical orbital separation is about 10 Hill radius, which only weakly depends on the mass and the semimajo r axis of protoplanets. We explain how this self-organized protoplanet -planetesimal system forms. (C) 1998 Academic Press.