IR spectroscopic study of olivine, enstatite and diopside irradiated with low energy H+ and He+ ions

In this article we investigate the interaction of silicate grains with light atoms ionized and accelerated during the propagation of shock waves in the diffuse interstellar medium (ISM). Such an interaction which is equivalent to the irradiation of the grains with accelerated ions, is a potentially important process for silicate grain evolution in the ISM. We present the results of irradiation experiments aimed at simulating this process. The same crystalline silicates as those identified around evolved stars before their injection in the ISM, forsterite, enstatite and diopside, were irradiated with light ions (H+, He+) at low energies (10-50 keV). The IR spectroscopic characteristics of the samples were analyzed before and after the irradiation in the 2-35 mum range allowing us to study the structural modifications in the irradiated minerals. The experiments show that low energy H+ (10 keV) and He+ (less than or equal to50 keV) ions efficiently amorphize crystalline silicates with fluence less than or equal to10(18) ions/cm(2). Since these experimental conditions are compatible with interstellar environments, the interaction of grains with high velocity shock waves may be responsible for the absence of crystalline silicates in the ISM. The comparison of the IR spectra of the irradiated silicates with observations of the Galactic Center is presented. This comparison calls into question the classical assignment of the interstellar amorphous silicate bands.