Star formation in high pressure, high energy density environments: Laboratory experiments of ISM dust analogs

Dust grains control the chemistry and cooling, and thus the gravitational collapse of interstellar clouds. Energetic particles, shocks and ionizing radiation can have a profound influence on the structure, lifetime and chemical reactivity of the dust, and therefore on the star formation efficiency. This would be especially important in forming galaxies, which exhibit powerful starburst (supernovae) and AGN (active galactic nucleus) activity. How dust properties are affected in such environments may be crucial for a proper understanding of galaxy formation and evolution. We present the results of experiments at LLNL which show that irradiation of the interstellar medium (ISM) dust analog forsterite (Mg₂SiO₄) with swift heavy ions (10 MeV Xe) and a large electronic energy deposition amorphizes its crystalline structure, without changing its chemical composition. From our data we predict that silicate grains in the ISM, even in dense and cold giant molecular clouds, can be amorphized by heavy cosmic rays (CR's). This might provide an explanation for the observed absence of crystalline dust in the ISM clouds of our Milky Way galaxy. This processing of dust by CR's would be even more important in forming galaxies and galaxies with active black holes.