Coagulation, fragmentation and radial motion of solid particles in protoplanetary disks

TLDR

The growth of solid particles toward meter sizes in protoplanetary disks must overcome rapid radial drift and destructive fragmentation. The study aims to improve dust‑particle coagulation algorithms and to test whether increasing the dust‑to‑gas ratio enables kilometer‑sized bodies to form despite radial drift and fragmentation. The authors perform numerical simulations that progressively include particle growth, radial and vertical motion, and fragmentation, while accelerating the coagulation scheme by a factor of ~10⁴. The results show that doubling the dust‑to‑gas ratio allows km‑sized bodies to form within 10⁴ yr inside 2 AU, but collisional fragmentation limits growth unless unrealistically high threshold velocities (>30 m/s) are used, and less than 5 % of small dust survives after 1 Myr regardless of fragmentation.

Abstract

The growth of solid particles towards meter sizes in protoplanetary disks has to circumvent at least two hurdles, namely the rapid loss of material due to radial drift and particle fragmentation due to destructive collisions. In this paper, we present the results of numerical simulations with more and more realistic physics involved. Step by step, we include various effects, such as particle growth, radial/vertical particle motion and dust particle fragmentation in our simulations. We demonstrate that the initial dust-to-gas ratio is essential for the particles to overcome the radial drift barrier. If this value is increased by a factor of 2 compared with the canonical value for the interstellar medium, km-sized bodies can form in the inner disk (<2 AU) within 104 yrs. However, we find that solid particles get destroyed through collisional fragmentation. Only with the unrealistically high-threshold velocities needed for fragmentation to occur (>30 m/s), particles are able to grow to larger sizes in disks with low α values. We also find that less than 5% of the small dust grains remain in the disk after 1 Myr due to radial drift, no matter whether fragmentation is included in the simulations or not. In this paper, we also present considerable improvements to existing algorithms for dust-particle coagulation, which speed up the coagulation scheme by a factor of ~ 104.