  We investigate planetesimal accretion via a direct N-body simulation of an
annulus at 1 AU orbiting a 1 $M_{\odot}$ star. The planetesimal ring, which
initially contains N = $10^6$ bodies is evolved into the oligarchic growth
phase. Unlike previous lower resolution studies, we find that the mass
distribution of planetesimals develops a bump at intermediate mass after the
oligarchs form. This feature marks a boundary between growth modes. The
smallest planetesimals are packed tightly enough together to populate mean
motion resonances with the oligarchs, which heats the small bodies, enhancing
their growth. If we depopulate most of the resonances by decreasing the width
of the annulus, this effect becomes weaker. To clearly demonstrate the dynamics
driving these growth modes, we also examine the evolution of a planetary embryo
embedded in an annulus of collisionless planetesimals. In this case, we find
that the resonances push planetesimals away from the embryo, decreasing the
surface density of the bodies adjacent to the embryo. This effect only occurs
when the annulus is wide enough and the mass resolution of the planetesimals is
fine enough to populate the resonances. The bump we observe in the mass
distribution resembles the 100 km power law break seen in the size distribution
of asteroid belt objects. Although the bump produced in our simulations occurs
at a size larger than 100 km, we show that the bump location is sensitive to
the initial planetesimal mass, which implies that this feature is potentially
useful for constraining planetesimal formation models.
