The Effect of Heavy Cosmic-Ray Ions on Silicate Grains in the Interstellar Dust

Astronomy and Astrophysics – Astronomy

Scientific paper

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Cosmic Rays, Dust, Extinction

Scientific paper

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 56 MeV Fe ions, while no tracks were induced by a 48 MeV 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 40-140 MeV, 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.

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