Collisions between carbonaceous grains in the interstellar medium
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 km/s 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. As a consequence, the energy left in the bulk is insufficient for shattering or even thermal processing. Grain size distribution is hardly affected by such collisions. These results cast doubt on the notion that collisions reduce grain lifetime so much as to require efficient growth processes in the interstellar medium.