Optimized simulation of interstellar grain-surface chemistry
This simulation corresponds to a more optimal implementation of the Monte Carlo chemical kinetics model outlined on the “videos” page. This trial simulation assumes a gas density of 108 cm-3, and a grain 10 atoms in diameter.
Each frame of the video corresponds to the formation of 10 water molecules; 10,000 surface water molecules are formed by the end of the simulation. Many more hydrogen molecules (H2) are also formed; most of these evaporate, but some are retained in the stronger surface-binding sites. Some oxygen molecules (O2) are also present in the ice mantle.
In spite of the very regular structure of the underlying grain, the ice mantle grows in a fairly smooth but irregular shape, forming crevices and a few small pores.
At current optimization, this simulation takes around 25 minutes to run.
Longer simulation of interstellar grain-surface chemistry
This video shows a simulation with the same input characteristics, but using a different random-number seed. Each frame of this video corresponds to the formation of 100 water molecules; 100,000 surface water molecules are formed by the end of the simulation.
The creviced structure is much more visible in this simulation.
The video below shows a cross-section of the final ice mantle shown in the video above, as it rotates. Particles that fall within a plane two molecules in depth are shown. The creviced/porous structure of the ice mantle may be seen more clearly using this cross-sectional view. Pores and crevices are seen to go deep within the mantle, sometimes to within a few layers of the grain surface itself. In the upper section of the mantle, approximately one grain-diameter above the grain itself, may be seen a closed pore.
Chemistry on large dust grains
Below is a much larger grain — still spherical, but with a rough surface.
To produce this, firstly, a smooth grain of radius 100 atoms was generated, using 257,900 particles to produce a closed outer shell. A further 10,000 particles were then randomly accreted (without diffusion) onto this surface, to produce the roughness.
…And yes, it looks like the Death Star.
Here is a preliminary video showing the build up of ice on this dust grain, assuming a gas density of 106 cm-3.
This simulation and some variations on that set-up are running currently. This will provide information about the formation of ice structure through surface-chemical processes, and how both the chemistry and structure together are affected by temperature, gas density, etc. Future simulations will also investigate different grain morphologies and roughnesses, as well as a wider range of chemical species.
Look for the journal publication soon…