Collisions between carbonaceous grains in the interstellar medium

Abstract

Semi-empirical molecular dynamics is used to simulate hydrocarbon grain sputtering and collisions which are extremely difficult to study experimentally. This microscopic and dynamic approach is particularly suited to high velocity impacts, where target destruction occurs far from equilibrium. A wide variety of processes are encountered, depending on grain size and velocity: vaporization, fragmentation, atomic implantation, sticking, elastic recoil, atomic chemisorption, H abstraction and H2 formation, etc. The impact-velocity threshold is about 10 kms-1 but complete grain destruction requires much higher velocities and nearly equal grain sizes. The main outcome of strong collisions is vaporization, i.e. formation of small molecules rather than solid fragments. Most of the impact energy is carried away by these molecules in the form of kinetic energy, with the consequence that the destruction efficiency of collisions is considerably reduced.