Molecular Dynamics Simulation of a Dust-Core Formation Stage in Space: H2O Formation on Surface Mg Sites at Around 3500 K

The dust formation process was simulated by a molecular dynamics method for the system consisting of Si, Mg, Al, O, H, and He in roughly cosmic abundance. As the temperature was lowered from 6000 K, the coagulation due to random collisions made the clusters grow, and solidification proceeded to make amorphous Mg-Al-silicate clusters of various sizes having an average coordination number of 2.2 for Si at 1000 K. Water formation was observed at around 3500 K on the surface Mg atoms that have been squeezed out from the inside due to thermal vibrations. These water molecules in the gas phase at 500 K might turn out to be the ice mantle at very cold temperatures that covers the thus formed dust.