Volume 538, February 2012
|Number of page(s)||17|
|Published online||14 February 2012|
Grain charging in protoplanetary discs
Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
Received: 29 September 2011
Accepted: 26 November 2011
Context. 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 magnetohydrodynamic (MHD) turbulence and grain growth, which are coupled in a two-way process.
Aims. 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 Df = 2.
Methods. 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.
Results. 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 N1/6 in the ion-dust limit. We find that grain charging in a non-stationary disc environment is expected to lead to similar results.
Conclusions. 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.
Key words: accretion, accretion disks / circumstellar matter / protoplanetary disks / stars: pre-main sequence
© ESO, 2012
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