From Dust to Planets: Simulating Particle Agglomeration in Protoplanetary Disks

Joseph Barranco, San Francisco State University

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This research project involves molecular dynamics simulations of fractal dust grains in a very dilute gas in the context of understanding how microscopic grains grow in a protoplanetary disk to seed the initial stages of planet formation. Specific project goals include: (i) Simulating the aerodynamic drag on fractal dust grains in a very dilute gas. In modeling, researchers typically employ drag laws for perfectly smooth spheres in a dense medium. However, in a 100 cubic meters, there may only be a few dust grains surrounded by a few kilograms of hydrogen and helium gas. To date, no one has calculated the aerodynamic drag on fractal dust grains. (ii) Simulating collisions of fractal dust grains to determine sticking probabilities. Hard spheres bounce off of each other, but fractal grains can compactify, which dissipates kinetic energy and converts it to heat. Dissipating collisional kinetic energy is crucial if grains are to stick and grow versus bouncing. (iii) Simulating the deposition of thin icy mantles on fractal dust grains. It has been hypothesized that grains may stick more readily if coated with an icy mantle. However, no calculations of this process have yet been done. Molecular Dynamics can help elucidate how water vapor may freeze onto dust grains to form an icy mantle, and then how subsequent collisions may lead to phase changes and refreezing which may increase the probability of sticking.