The destruction and growth of dust grains in interstellar space - I. Destruction by sputtering

Abstract

The processes governing the destruction and growth of dust grains in interstellar space are investigated with a view to establishing the conditions required for the existence of ice mantles. In this paper sputtering by particles with energies in the eV to GeV range is considered. Previous sputtering yield estimates which were based on theoretical considerations are shown to be greatly in error for incident particle energies less than 1 keV. Empirical formulae for the sputtering threshold energy and the sputtering yield are derived from the extensive experimental data available. The sputtering of grains in HII regions, in the intercloud medium, and in shock waves produced by cloud-cloud collisions and by supernova remnants is investigated. Of these, supernova remnants are shown to be the most important, leading to lifetimes of aproximately 2x10^8 yrs for ice grains and between 5-20x10^8 yrs for refractory grains. Destruction rates are estimated for grains bombarded by MeV and GeV cosmic rays. It is shown that collision cascade sputtering dominates evaporative sputtering produced by thermal spikes. It is also shown that even if all the electron excitation energy loss in a grain material could be transferred to the lattice particles, the observed cosmic ray flux spectrum could not cause significant destruction of ice grains.