  Formation models in which terrestrial bodies grow via the pairwise accretion
of planetesimals have been reasonably successful at reproducing the general
properties of the solar system, including small body populations. However,
planetesimal accretion has not yet been fully explored in the context of the
wide variety of recently discovered extrasolar planetary systems, particularly
those that host short-period terrestrial planets. In this work, we use direct
N-body simulations to explore and understand the growth of planetary embryos
from planetesimals in disks extending down to ~1 day orbital periods. We show
that planetesimal accretion becomes nearly 100 percent efficient at short
orbital periods, leading to embryo masses that are much larger than the
classical isolation mass. For rocky bodies, the physical size of the object
begins to occupy a significant fraction of its Hill sphere towards the inner
edge of the disk. In this regime, most close encounters result in collisions,
rather than scattering, and the system does not develop a bimodal population of
dynamically hot planetesimals and dynamically cold oligarchs, like is seen in
previous studies. The highly efficient accretion seen at short orbital periods
implies that systems of tightly-packed inner planets should be almost
completely devoid of any residual small bodies. We demonstrate the robustness
of our results to assumptions about the initial disk model, and also
investigate the effects that our simplified collision model has on the
emergence of this non-oligarchic growth mode in a planet forming disk.

Earth and Planetary Astrophysics