Particle Evolution and Planetesimals

Pan, Padoan et al. 2011, ApJ 740, 6

Dust grains with micrometer to millimeter size are an important component of many astrophysical environments and, most significantly, of protoplanetary disks. Although they contain a small mass fraction (ap- proximately 1% on average), solid particles affect the gas dynamics and emission through various processes such as thermal exchange, surface chemistry, and radiative transfer. In protoplanetary disks, their migration, sedimentation, and collisional coalescence and fragmentation set the stage for planet forma- tion, as these processes are believed to lead to the formation of planetesimals. Solid particles are dragged by gas motions, which are generally turbulent in astrophysical systems. The drag force of the gas turbulence, along with the generic feature that the inertial particle trajectories are dissipative, gives rise to a complex non-linear dynamics consisting of stochastic accelerations and decelerations, resulting in motions that partially reflect features of the velocity field of the gas that carries the particles. The most important result of this complex dynamics is the spatial clustering of dust grain. The strong particle density enhancements may help accelerate the formation of planetesimals.