Impacts into rotating targets: angular momentum draining and efficient formation of synthetic families

About 10% of the observed asteroids have rotational periods lower than P = 3 h and seem to be relatively close to the spin barrier. Yet, the rotation has often been neglected in simulations of asteroid collisions. To determine the effect of rotation, we performed a large number of impact simulations with rotating targets. We developed a new unified smoothed particle hydrodynamics and N-body code with self-gravity, suitable for simulations of both fragmentation phase and gravitational reaccumulation. The code has been verified against previous ones, but we also tested new features, such as rotational stability, tensile stability, etc. Using the new code, we ran simulations with D_pb = 10 and 100 km monolithic targets and compared synthetic asteroid families created by these impacts with families corresponding to non-rotating targets. The rotation affects mostly cratering events at oblique impact angles. The total mass ejected by these collisions can be up to five times larger for rotating targets. We further computed the transfer of the angular momentum and determined conditions under which impacts accelerate or decelerate the target. While individual cratering collisions can cause both acceleration and deceleration, the deceleration prevails on average. Collisions thus cause a systematic spin-down of the asteroid population.