The evolution of grain mantles and silicate dust growth at high redshift
  In dense molecular clouds, interstellar grains are covered by mantles of iced
molecules. The formation of the grain mantles has two important consequences:
it removes species from the gas phase and promotes the synthesis of new
molecules on the grain surfaces. The composition of the mantle is a strong
function of the environment which the cloud belongs to. Therefore, clouds in
high-zeta galaxies, where conditions -like temperature, metallicity and cosmic
rays flux- are different from those in the Milky Way, will have different grain
mantles. In the last years, several authors have suggested that silicate grains
might grow by accretion of silicon bearing species on smaller seeds. This would
occur simultaneously to the formation of the iced mantles and be greatly
affected by its composition as a function of time. In this work, we present a
numerical study of the grain mantle formation in high-zeta galaxies and we
quantitatively address the possibility of silicate growth. We find that the
mantle thickness decreases with increasing redshift, from about 120 to 20
layers for z varying from 0 to 8. Furthermore, the mantle composition is also a
strong function of the cloud redshift, with the relative importance of CO, CO2,
ammonia, methane and methanol highly varying with z. Finally, being Si-bearing
species always a very minor component of the mantle, the formation of silicates
in molecular clouds is practically impossible.
