Monte Carlo Simulations of the Formation and Morphology of Interstellar Ices

The porosity present in ices contains many clues on the conditions of their formation and evolution, as well as their trapping capacities. Laboratory experiments studying such porosity are essential to better understand interstellar ices. In this work, we present recent theoretical results on water and CO ices that aim at reproducing laboratory experiments. We use Monte Carlo methods in order to describe the fate of each molecule present in the ices, which is determined by the different processes that molecules can experience, such as accretion, diffusion and evaporation. For water ices, we show that porosity depends on the conditions of deposition, and that with increasing temperature, small pores connect and merge, allowing trapped molecules to meet and react within the pores network and thus increasing the efficiency of the reaction. For CO ices, we show that deposition conditions can also give different types of CO ices, with many weakly bound CO molecules as the deposition occurs at very low temperatures. The presence of weakly bound CO molecules could explain the far less important freeze out seen in pre-stellar cores, compared to what is commonly expected.