Size Dependence of the Bouncing Barrier in Protoplanetary Dust Growth

1. Key Points

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Numerical simulations using soft-sphere discrete element methods of the collision of two solid microparticle aggregates were performed, and the probability of two aggregates sticking together and forming a larger aggregate after the collision (referred to as “sticking probability”) was investigated using various size of aggregates consist of approximately 10,000 to 140,000 particles.

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For the first time in computational investigations, it was observed that the sticking probability decreased as the radius of colliding aggregates increased.

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The findings obtained in this study will not only deepen our understanding of the fundamental physics, but provide insights into the planet formation process from cosmic dust. It suggests that the direct collisional growth of cosmic dust microparticles to form planetesimals (*1), which are the seeds of planets, would be hindered by the collisional bouncing.

2. Overview

Dr. Sota Arakawa, a Young Research Fellow in the Center for Mathematical Science and Advanced Technology, JAMSTEC and his colleagues used the PC cluster at the National Astronomical Observatory of Japan (NAOJ) (*2) to investigate the collision behavior of solid microparticle aggregates, which are the building blocks of planets, through numerical simulations using soft-sphere discrete element methods. By conducting numerical simulations on aggregates of various sizes, they revealed that the probability of two aggregates sticking after collision decreases when the aggregates are larger.

Planetesimals, which are "seeds" for planets, are believed to form through multiple collisions and mergers of solid microparticles in protoplanetary disks (*3). However, there was a unsolved major question, what conditions two aggregates of microparticles stick together. The result suggests that as the aggregates become larger, they become less likely to stick together after collision, indicating that the formation of planetesimals through direct collisional growth of aggregates is challenging. This provides significant insights for understanding how planets formed.

These findings were published in “The Astrophysical Journal Letters” on July 5^th. This research was supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (KAKENHI) JP22J00260, JP22KJ1292, and JP18H05438.

Fig. Evolution from microparticles to planets in protoplanetary disks