1112.2692 (Martin Ilgner)
Martin Ilgner
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.
View original:
http://arxiv.org/abs/1112.2692
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