When Two Worlds Collide: How Collisions Between Silicate and Carbonaceous Nanograins can Drive Complex Chemistry

Abstract

Our overall objective is to determine how collision velocity governs the interactions between silicate and carbonaceous nanograins. We aim to: i) identify collisional regimes capable of producing mixed silicate-carbonaceous grains and/or chemically complex molecular species, and ii) quantify fragmentation threshold velocities related to dust destruction. We performed molecular dynamics simulations employing a machine-learning force field to model head-on collisions between silicate and carbonaceous nanograins of comparable masses. Collision velocities span 1 - 11 km/sec. For all collisions, we tracked the extent of grain-grain mixing and the formation of molecular fragments. We identified four velocity regimes: 1) <1.5 km/sec, where grains bounce off one another, 2) 1.5-3.5 km/sec, where sticking between the grains starts to occur, 3) 3.5-7.5 km/sec, where grains tend to aggregate and form inter-grain chemical bonds, yielding stable mixed grains, and 4) >7.5 km/sec, 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 7.5 km/sec. 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.