Ionization and Dust Charging in Protoplanetary Disks

TL;DR

This paper introduces a self-consistent analytical model to accurately calculate charged species abundances in dusty protoplanetary disks, improving understanding of ionization, dust charging, and magnetic coupling crucial for disk evolution.

Contribution

The paper presents a novel analytical model that accounts for grain surface recombination and arbitrary grain sizes, enhancing the accuracy of dust charging predictions in protoplanetary disks.

Findings

Conventional low grain charge approximation is often invalid in typical disks.

The model's predictions impact understanding of dust coagulation and dead zone development.

Effective ion mass has minimal influence on charged species abundances.

Abstract

Ionization-recombination balance in dense interstellar and circumstellar environments is a key factor for a variety of important physical processes, such as chemical reactions, dust charging and coagulation, coupling of the gas with magnetic field and development of instabilities in protoplanetary disks. We determine a critical gas density above which the recombination of electrons and ions on the grain surface dominates over the gas-phase recombination. For this regime, we present a self-consistent analytical model which allows us to exactly calculate abundances of charged species in dusty gas, without making assumptions on the grain charge distribution. To demonstrate the importance of the proposed approach, we check whether the conventional approximation of low grain charges is valid for typical protoplanetary disks, and discuss the implications for dust coagulation and development of the "dead zone" in the disk. The presented model is applicable for arbitrary grain-size distributions and, for given dust properties and conditions of the disk, has only one free parameter - the effective mass of the ions, shown to have a low effect on results. The model can be easily included in numerical simulations following the dust evolution in dense molecular clouds and protoplanetary disks.