  In this paper we extend our numerical method for simulating terrestrial
planet formation from Leinhardt and Richardson (2005) to include dynamical
friction from the unresolved debris component. In the previous work we
implemented a rubble pile planetesimal collision model into direct N-body
simulations of terrestrial planet formation. The new collision model treated
both accretion and erosion of planetesimals but did not include dynamical
friction from debris particles smaller than the resolution limit for the
simulation. By extending our numerical model to include dynamical friction from
the unresolved debris, we can simulate the dynamical effect of debris produced
during collisions and can also investigate the effect of initial debris mass on
terrestrial planet formation. We find that significant initial debris mass, 10%
or more of the total disk mass, changes the mode of planetesimal growth.
Specifically, planetesimals in this situation do not go through a runaway
growth phase. Instead they grow concurrently, similar to oligarchic growth. In
addition to including the dynamical friction from the unresolved debris, we
have implemented particle tracking as a proxy for monitoring compositional
mixing. Although there is much less mixing due to collisions and gravitational
scattering when dynamical friction of the background debris is included, there
is significant inward migration of the largest protoplanets in the most extreme
initial conditions.
