Sputtering of grains in C-type shocks

Abstract

Sputtering yields are reported for the release of Mg, Fe, Si and O under impact of He, C, O, Si and Fe on grain material composed of Mg- and Fe-bearing silicates. The yields were derived using the TRIM code, which simulates the results of the transport of ions in matter by means of classical Monte Carlo techniques. The energetics of the sputtering process are a key factor in the sputtering calculations, and so detailed determinations have been made of the energy with which atoms are bound to the lattice, using solid-state simulation programs. The probability of ejection of an atom is computed at a given energy, for a number of angles of incidence, and integrated to obtain the mean yield at that energy. These numerical results are then fitted with a simple function of energy for convenience in subsequent applications. A grid of C-type shock models has been computed, using our new sputtering yields, for pre-shock densities in the range 104 ≤ nH = n(H) + 2n(H2) ≤ 106 cm-3 and shock speeds 20 ≤ υs ≤ 45 km s-1. Sputtered fractions can be high, exceeding 50 per cent for shock speeds in excess of approximately 40 km s-1. The column densities of Si and SiO were also computed, for comparison with observations.