  The final stage of terrestrial planet formation is known as the giant impact
stage where protoplanets collide with one another to form planets. So far this
stage has been mainly investigated by N-body simulations with an assumption of
perfect accretion in which all collisions lead to accretion. However, this
assumption breaks for collisions with high velocity and/or a large impact
parameter. We derive an accretion condition for protoplanet collisions in terms
of impact velocity and angle and masses of colliding bodies, from the results
of numerical collision experiments. For the first time, we adopt this realistic
accretion condition in N-body simulations of terrestrial planet formation from
protoplanets. We compare the results with those with perfect accretion and show
how the accretion condition affects terrestrial planet formation. We find that
in the realistic accretion model, about half of collisions do not lead to
accretion. However, the final number, mass, orbital elements, and even growth
timescale of planets are barely affected by the accretion condition. For the
standard protoplanetary disk model, typically two Earth-sized planets form in
the terrestrial planet region over about 100 M years in both realistic and
perfect accretion models. We also find that for the realistic accretion model,
the spin angular velocity is about 30% smaller than that for the perfect
accretion model that is as large as the critical spin angular velocity for
rotational instability. The spin angular velocity and obliquity obey Gaussian
and isotropic distributions, respectively, independently of the accretion
condition.
