Effects of size, composition, and volatile coverage on the fragmentation velocity of granular clusters in protoplanetary disks

Context . Interstellar dust grains accrete into protoplanetary disks during star formation. The coagulation of these grains produces dust clusters that collide with each other. The ability of the grains and clusters to stick depends on the surface free energy of the grains, which, in turn, depends on their composition, temperature, and volatile coverage. If the clusters do not fragment on collision, they can grow large enough to be affected by the streaming instability and undergo gravitational collapse into planetesimals. Aims . The goal is to determine the critical velocity, V frag , above which dust clusters fragment across a range of parameters. Methods . We used the granular dynamics version of the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) to simulate the collision of spherical clusters of equal-sized grains at varying impact velocities to determine V frag . We constructed the initial clusters using the AGregate GENeration Tool (AGGENT) to determine their filling factor and size. We further varied the size and surface free energy of the grains and the impact parameter of the collision. Results . We determined the dependence of V frag on all parameters tested. An empirical fit to our numerical results reproduced them to better than 22% over a range of collision velocities 1–300 m s −1 . We validated the fit by extrapolating them to a regime that is directly comparable to experimental results. Conclusions . The properties of grains vary strongly with radial position in protoplanetary disks. Therefore, we recommend the use of our empirical fit to V frag in large-scale models of dust coagulation and planetesimal formation, rather than assuming a constant value.