Grain charging in protoplanetary discs

Abstract

Recent work identified a growth barrier for dust coagulation that originates in the electric repulsion between colliding particles. Depending on its charge state, dust material may have the potential to control key processes towards planet formation such as MHD (magnetohydrodynamic) turbulence and grain growth which are coupled in a two-way process. We quantify the grain charging at different stages of disc evolution and differentiate between two very extreme cases: compact spherical grains and aggregates with fractal dimension D_f = 2. Applying a simple chemical network that accounts for collisional charging of grains, we provide a semi-analytical solution. This allowed us to calculate the equilibrium population of grain charges and the ionisation fraction efficiently. The grain charging was evaluated for different dynamical environments ranging from static to non-stationary disc configurations. The results show that the adsorption/desorption of neutral gas-phase heavy metals, such as magnesium, effects the charging state of grains. The greater the difference between the thermal velocities of the metal and the dominant molecular ion, the greater the change in the mean grain charge. Agglomerates have more negative excess charge on average than compact spherical particles of the same mass. The rise in the mean grain charge is proportional to N**(1/6) in the ion-dust limit. We find that grain charging in a non-stationary disc environment is expected to lead to similar results. The results indicate that the dust growth and settling in regions where the dust growth is limited by the so-called "electro-static barrier" do not prevent the dust material from remaining the dominant charge carrier.

18 pages, 10 figures, accepted for publication in Astronomy and Astrophysics

References in corpus (4)

• Heat and Dust in Active Layers of Protostellar Disks
• Modeling Magnetorotational Turbulence in Protoplanetary Disks with Dead Zones
• Turbulent transport and its effect on the dead zone in protoplanetary discs
• On the dynamics of planetesimals embedded in turbulent protoplanetary discs with dead zones

Cited by in corpus (12)

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• Ionization and Dust Charging in Protoplanetary Disks
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• The Nonlinear Ohm's Law: Plasma Heating by Strong Electric Fields and its Effects on the Ionization Balance in Protoplanetary Disks
• Inhibited Coagulation of Micron-size Dust Due to the Electrostatic Barrier
• Electric heating and angular momentum transport in laminar models of protoplanetary disks
• Explaining Mercury's Density through Magnetic Erosion
• Observation of bottom-up formation for charged grain aggregates related to pre-planetary evolution beyond the bouncing barrier
• Magnetohydrodynamics of protoplanetary discs
• Ionisation and discharge in cloud-forming atmospheres of brown dwarfs and extrasolar planets
• Vertical probe-induced asymmetric dust oscillation in complex plasma