  The final stage of planet formation is dominated by collisions between
planetary embryos. The dynamics of this stage determine the orbital
configuration and the mass and composition of planets in the system. In the
solar system, late giant impacts have been proposed for Mercury, Earth, Mars,
and Pluto. In the case of Mercury, this giant impact may have significantly
altered the bulk composition of the planet. Here we present the results of
smoothed particle hydrodynamics simulations of high-velocity (up to ~5 v_esc)
collisions between 1 and 10 M_Earth planets of initially terrestrial
composition to investigate the end stages of formation of extrasolar
super-Earths. As found in previous simulations of collisions between smaller
bodies, when collision energies exceed simple merging, giant impacts are
divided into two regimes: (1) disruption and (2) hit-and-run (a grazing
inelastic collision and projectile escape). Disruption occurs when the impact
parameter is near zero, when the projectile mass is small compared to the
target, or at extremely high velocities. In the disruption regime, we derive
the criteria for catastrophic disruption (when half the total colliding mass is
lost), the transition energy between accretion and erosion, and a scaling law
for the change in bulk composition (iron-to-silicate ratio) resulting from
collisional stripping of a mantle.
