Growth of Settling Dust Particles in Turbulent Disks
First Author:
Taku Takeuchi
Email: taku AT kobe-u.ac.jp
Kobe University
Rokkodai-cho 1-1
Nada-ku
Kobe 657-8501 Japan
Abstract
Dust growth and settling in turbulent disks are studied by solving a coagulation equation. We consider dust particles that settle toward the disk midplane, and solved a coagulation equation. We assumed that the disk has weak turbulence (alpha ~10-5), and that dust collisions always result in coagulation without any fragmentation. We investigated when the midplane dust layer forms. The formation time of the dust layer depends on what size we pay attention. The first generation of particles grows during their sedimentation. Even after the first generation has settled on the midplane, there remains some dust at high altitudes, and they continue to fall onto the dust layer. Suppose we focus on a specific size of the dust. The formation of the dust layer of that specific size completes when the dust materials above the layer deplete. For the dust particles to grow larger than a certain size during sedimentation, it is required that an enough amount of the dust remains above the layer. This required amount of the dust is larger for larger particles to grow. After the dust above the layer depletes, the structure of the layer settles to the steady one that determined by the balance between sedimentation and turbulent diffusion, that can be obtained analytically. For particles larger than 1 cm, the dust layer always has the steady structure, even when the first generation appears at the midplane. We compared the dust growth timescale to that of dust radial drift. For the dust to grow larger than the critical size at that the particles drift fastest, the turbulence must be weak (alpha < 10-7) or the initial dust/gas ratio is large (> 0.1).
