860 research outputs found
Harvesting the decay energy of Al to drive lightning discharge in protoplanetary discs
Chondrules in primitive meteorites likely formed by recrystallisation of dust
aggregates that were flash-heated to nearly complete melting. Chondrules may
represent the building blocks of rocky planetesimals and protoplanets in the
inner regions of protoplanetary discs, but the source of ubiquitous thermal
processing of their dust aggregate precursors remains elusive. Here we
demonstrate that escape of positrons released in the decay of the short-lived
radionuclide Al leads to a large-scale charging of dense pebble
structures, resulting in neutralisation by lightning discharge and
flash-heating of dust and pebbles. This charging mechanism is similar to a
nuclear battery where a radioactive source charges a capacitor. We show that
the nuclear battery effect operates in circumplanetesimal pebble discs. The
extremely high pebble densities in such discs are consistent with conditions
during chondrule heating inferred from the high abundance of sodium within
chondrules. The sedimented mid-plane layer of the protoplanetary disc may also
be prone to charging by the emission of positrons, if the mass density of small
dust there is at least an order of magnitude above the gas density. Our results
imply that the decay energy of Al can be harvested to drive intense
lightning activity in protoplanetary discs. The total energy stored in positron
emission is comparable to the energy needed to melt all solids in the
protoplanetary disc. The efficiency of transferring the positron energy to the
electric field nevertheless depends on the relatively unknown distribution and
scale-dependence of pebble density gradients in circumplanetesimal pebble discs
and in the protoplanetary disc mid-plane layer.Comment: Submitted to Astronomy & Astrophysics, 22 pages, revised version in
response to referee repor
The fate of planetesimals in turbulent disks with dead zones. II. Limits on the viability of runaway accretion
A critical phase in the standard model for planet formation is the runaway
growth phase. During runaway growth bodies in the 0.1--100 km size range
(planetesimals) quickly produce a number of much larger seeds. The runaway
growth phase is essential for planet formation as the emergent planetary
embryos can accrete the leftover planetesimals at large gravitational focusing
factors. However, torques resulting from turbulence-induced density
fluctuations may violate the criterion for the onset of runaway growth, which
is that the magnitude of the planetesimals' random (eccentric) motions are less
than their escape velocity. This condition represents a more stringent
constraint than the condition that planetesimals survive their mutual
collisions. To investigate the effects of MRI turbulence on the viability of
the runaway growth scenario, we apply our semi-analytical recipes of Paper I,
which we augment by a coagulation/fragmentation model for the dust component.
We find that the surface area-equivalent abundance of 0.1 micron particles is
reduced by factors 10^2--10^3, which tends to render the dust irrelevant to the
turbulence. We express the turbulent activity in the midplane regions in terms
of a size s_run above which planetesimals will experience runaway growth. We
find that s_run is mainly determined by the strength of the vertical net field
that threads the disks and the disk radius. At disk radii beyond 5 AU, s_run
becomes larger than ~100 km and the collision times among these bodies longer
than the duration of the nebula phase. Our findings imply that the classical,
planetesimal-dominated, model for planet formation is not viable in the outer
regions of a turbulent disk.Comment: ApJ accepte
Grain charging in protoplanetary discs
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.Comment: 18 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Electrostatic Barrier against Dust Growth in Protoplanetary Disks. I. Classifying the Evolution of Size Distribution
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.Comment: 20 pages, 22 figures, accepted for publication in Ap
Charging and coagulation of dust in protoplanetary plasma environments
Combining a particle-particle, particle-cluster and cluster-cluster
agglomeration model with an aggregate charging model, the coagulation and
charging of dust particles in various plasma environments relevant for
proto-planetary disks have been investigated. The results show that charged
aggregates tend to grow by adding small particles and clusters to larger
particles and clusters, leading to greater sizes and masses as compared to
neutral aggregates, for the same number of monomers in the aggregate. In
addition, aggregates coagulating in a Lorentzian plasma (containing a larger
fraction of high-energy plasma particles) are more massive and larger than
aggregates coagulating in a Maxwellian plasma, for the same plasma densities
and characteristic temperature. Comparisons of the grain structure, utilizing
the compactness factor, {\phi}{\sigma}, demonstrate that a Lorentzian plasma
environment results in fluffier aggregates, with small {\phi}{\sigma}, which
exhibit a narrow compactness factor distribution. Neutral aggregates are more
compact, with larger {\phi}{\sigma}, and exhibit a larger variation in
fluffiness. Measurement of the compactness factor of large populations of
aggregates is shown to provide information on the disk parameters that were
present during aggregation
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