The effect of heavy cosmic-ray ions on silicate grains in the interstellar dust

Abstract

Electronmicroscopic samples of crystalline Mg2SiO4 forsterite were irradiated by energetic Ar, Fe, Kr, and Xe ions at room temperature. Tracks with a mean radius Re = 1.36 nm were observed after irradiation by 56MeV Fe ions, while no tracks were induced by a 48MeV Ar beam. Amorphization of forsterite grains by cosmic-ray (CR) Fe ions are discussed, including the effects of low temperature, ion velocity, and ion-induced crystallization. CR Fe ions induce amorphous tracks in crystalline forsterite only in the range 40-140MeV, and the period of time for complete amorphization is τcr ≈ 13,400 Myr. Our estimate is τcr ≈ 1300 Myr for enstatite. Thus, heavy CR particles do not reduce the crystallinity of silicate grains within a reasonable time, as supposed previously. However, energetic ions can induce crystallization in amorphous solids, and this may be partially or fully responsible for the estimated 0.2% crystallinity of silicates in the interstellar medium. © 2010. The American Astronomical Society. All rights reserved.