Inhibited Coagulation of Micron-size Dust Due to the Electrostatic Barrier (2001.06340v1)

Abstract: The collisional evolution of solid material in protoplanetary disks is a crucial step in the formation of planetesimals, comets, and planets. Although dense protoplanetary environments favor fast dust coagulation, there are several factors that limit the straightforward pathway from interstellar micron-size grains to pebble-size aggregates. Apart from the grain bouncing, fragmentation, and fast drift to the central star, a notable limiting factor is the electrostatic repulsion of like-charged grains. In this study we aim at theoretical modeling of the dust coagulation coupled with the dust charging and disk ionization calculations. We show that the electrostatic barrier is a strong restraining factor to the coagulation of micrometer-size dust in dead zones of the disk (where the turbulence is suppressed). While the sustained turbulence helps to overcome the electrostatic barrier, low fractal dimensions of dust aggregates can potentially block their further coagulation even in this case. Coulomb repulsion may keep a significant fraction of small dust in the disk atmosphere and outer regions.

• The paper demonstrates that electrostatic repulsion significantly inhibits the coagulation of micron-sized dust grains in protoplanetary disks, especially in regions with low turbulence.
• Sufficient turbulence, particularly with an alpha parameter around 10 -3, can provide the necessary kinetic energy to overcome the electrostatic barrier and facilitate dust growth.
• The fractal dimension of dust aggregates influences this barrier, with more fractal grains facing greater electrostatic hindrance to their aggregation.

Overview of "Inhibited coagulation of micron-size dust due to the electrostatic barrier"

The paper, authored by V.V. Akimkin et al., explores the complex dynamic processes governing the coagulation of dust grains in protoplanetary disks. It addresses the critical barrier posed by electrostatic repulsion in the coagulation of micron-sized dust grains, which is a significant factor influencing the evolution of these disks towards planetesimal formation.

Summary

The primary focus of the paper is the role of electrostatic repulsion in hindering the coagulation of interstellar micron-size dust grains. This is a non-trivial issue since the collisional growth of solid material in protoplanetary disks is crucial for forming larger planetary bodies like planetesimals and eventually planets. Despite the dense environments of these disks potentially facilitating rapid dust coagulation, several factors—including grain bouncing, fragmentation, and rapid drift towards the central star—pose significant challenges. The electrostatic repulsion arising from like-charged grains adds another significant layer of complexity.

The authors conduct a theoretical modeling exercise that couples dust coagulation with charging dynamics and disk ionization calculations. They demonstrate that the electrostatic barrier can severely restrict the growth of micrometer-sized dust in "dead zones" of the disk, which are characterized by suppressed turbulence. These simulated conditions show that unless sufficient turbulence exists to provide the necessary kinetic energy to overcome the Coulomb barriers, coagulation may be significantly limited.

Key Findings

The paper presents numerical simulations using a model that incorporates local disk conditions to estimate when the electrostatic barrier is likely to be an impediment to dust growth. The results indicate:

• Electrostatic Repulsion as a Barrier: The simulations highlight the constraint posed by electrostatic repulsion on the coagulation process, especially for micron-sized particles.
• Impact of Turbulence: Where turbulence is adequate (particularly outside dead zones), it can provide the requisite kinetic energy to facilitate grain coagulation by overcoming Coulomb repulsion. The critical turbulence parameter, α, is notably effective when it is in the range of 10^{-3}.
• Fractality and Grain Growth: The fractal dimension of dust aggregates significantly influences the coagulation dynamics, with more fractal grains (those with lower fractal dimensions) experiencing more substantial electrostatic barriers, which tend to limit their growth.
• Practical and Theoretical Implications: This work has implications for understanding the distribution of dust grain sizes throughout a disk and for elucidating the ubiquitous presence of micron-sized grains, contributing to opacity and radiation pressure dynamics.

Implications for Future Research

This paper's findings lay the foundational understanding needed to explore more complex interactions in protoplanetary disks involving electrostatic interactions. Possible future developments in this research domain may include more comprehensive models that integrate global disk dynamics, radial and vertical dust drift, and grain fragmentation processes to better simulate real disk conditions. Moreover, the coupling of these dynamics with observational data of turbulence (through linewidth measurements) can offer more refined constraints on existing models.

Conclusion

The paper underlines the critical role of electrostatic repulsion in the early stages of planet formation within protoplanetary disks. By combining insights from ionization, dust charging, and aggregation physics, this research enhances our understanding of how micron-sized dust grains behave and aggregate under varying disk conditions. It opens avenues for subsequent research to provide more precise constraints and to refine planet formation theories further.