Electrostatic Barrier against Dust Growth in Protoplanetary Disks. I. Classifying the Evolution of Size Distribution

TL;DR

This paper investigates how electrostatic charging influences dust grain growth in protoplanetary disks, revealing conditions that lead to stalled growth or bimodal size distribution evolution, with implications for planet formation.

Contribution

It introduces a combined analysis of monodisperse and size-evolving dust growth models considering electrostatic effects and porosity, providing predictive criteria for dust evolution outcomes.

Findings

Electrostatic repulsion can halt monodisperse dust growth.

Bimodal growth occurs with a limited number of large aggregates.

Small aggregates can prevent large ones from charging strongly.

Abstract

Collisional growth of submicron-sized dust grains into macroscopic aggregates is the first step of planet formation in protoplanetary disks. These grains are expected to carry nonzero negative charges in the weakly ionized disks, but its effect on their collisional growth has not been fully understood so far. In this paper, we investigate how the charging affects the evolution of the dust size distribution properly taking into account the charging mechanism in a weakly ionized gas as well as porosity evolution through low-energy collisions. To clarify the role of the size distribution, we divide our analysis into two steps. First, we analyze the collisional growth of charged aggregates assuming a monodisperse (i.e., narrow) size distribution. We show that the monodisperse growth stalls due to the electrostatic repulsion when a certain condition is met, as is already expected in the previous work. Second, we numerically simulate dust coagulation using Smoluchowski's method to see how the outcome changes when the size distribution is allowed to freely evolve. We find that, under certain conditions, the dust undergoes bimodal growth where only a limited number of aggregates continue to grow carrying the major part of the dust mass in the system. This occurs because remaining small aggregates efficiently sweep up free electrons to prevent the larger aggregates from being strongly charged. We obtain a set of simple criteria that allows us to predict how the size distribution evolves for a given condition. In Paper II (arXiv:1009.3101), we apply these criteria to dust growth in protoplanetary disks.