Silicate and Carbonaceous collisions

Collision velocities span 1 to 11km/s . For all collisions, we tracked the extent of grain-grain mixing and the formation of molecular fragments. We identified four velocity regimes: 1) <~1.5km/s, where grains bounce off one another; 2) ~1.5-3.5km/s, where sticking between the grains starts to occur; 3) ~3.5-7.5km/s, where grains tend to aggregate and form inter-grain chemical bonds, yielding stable mixed grains; and 4) >~7.5km/s, where fragmentation dominates.

The latter regime produces CO as the main product, along with hydrocarbons, complex organic molecules, molecular silicates, and mixed carbonaceous-silicate clusters. The fragmentation threshold velocity for these collisions is approximately 7.5km/s. We show that collision velocities govern both the physical and chemical outcomes of silicate-carbonaceous nanograin interactions.

In the fragmentation regime, collisions provide a viable pathway for generating mixed grains and a wide range of molecular species, many of which have been observationally detected. Here, we provide a simple credible mechanism linking the physics of grain processing with observed complex interstellar chemistry.