Discrete element simulations of self-gravitating rubble pile collisions: the effects of non-uniform particle size and rotation

We present a new implementation of a soft-sphere discrete element code to simulate the dynamics of self-gravitating granular materials. The code is used to study the outcome of sub-sonic collisions between self-gravitating rubble piles with masses ranging from 6x10^{21} to 6x10^{22} g. These masses are representative of asteroids and planetesimals in the \(\sim100\)~km range. We simulate rubble piles composed of a range of particle sizes and analyze the collision outcomes focusing on the properties of the largest surviving fragment. We successfully test and validate the code against previous results. The results of our study show that rubble piles formed by collisions of two parent rubble piles do not maintain the same particle size distribution as their parents. Rubble piles formed in low velocity collisions are characterized by a larger fraction of large particles, while the largest fragments of high-velocity collisions show a decrease in their mean particle size. In both cases the effect is small, but could build up to a noticeable difference after multiple collisions. We ascribe this effect to the fact that large particles transmit most of the forces during the collisions. In addition, we find that the mass of the largest post-collision fragment depends on the rotation of the colliding rubble piles. This effect is especially noticeable when the pre-collision spin axes are parallel with each other and perpendicular to the relative velocity. This finding can be particularly relevant for meter to kilometer sized bodies embedded in protostellar accretion disks, where viscous stresses can efficiently align the target and projectile spin axes.