A Monte-Carlo coagulation-fragmentation model for the Pencil Code (Andras)

Link to the talk (pdf)

Try overcoming the meter sized barrier by coagulation instead of self-gravity

1g -> 1e12 monomers inside a particle. Not feasible with current computers.

Need something more statistical.

Critics to Ander's nature paper: starts with particles that are already too big (40-60-80)cm, and without dust physics (coagulation-fragmentation; CF). Can planet formation work with smaller particles and detailed CF models?

Method. Add virtual particles. Real particles grow by colliding with virtual particles. Real particles do not meet, the rate of collision real-real is much smaller than real-virtual. The model is just real-virtual. There are no virtual-virtual collision either.

Test: Smoluchowski equation that describes coagulation. The test reproduces the mass x number density distribution. For comparison, see Ormel et al. 2008, Wetherill 1990.

Important energies: Eroll-> Energy needed to rotate a monomer by 90 degrees.

Porosity model:

Ecoll < 5Eroll (Hit and stick)

If 5Eroll < Ecoll < Efrag (compaction)

Fragmentation always lead to a monomer (limitation)

With porosity, the radius gets bigger by a factor 2 compared to just using turbulence + brownian
motion.

For the model showed, the grains reach complete compactness at 0.1cm size. The MMSN reaches compactness just at m size.

The code seems fast. 1 million timesteps takes minutes in a single processor. Seems no overhead for the pencil code.