2,613 research outputs found
Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates
Porosity evolution of dust aggregates is crucial in understanding dust
evolution in protoplanetary disks. In this study, we present useful tools to
study the coagulation and porosity evolution of dust aggregates. First, we
present a new numerical method for simulating dust coagulation and porosity
evolution as an extension of the conventional Smoluchowski equation. This
method follows the evolution of the mean porosity for each aggregate mass
simultaneously with the evolution of the mass distribution function. This
method reproduces the results of previous Monte Carlo simulations with much
less computational expense. Second, we propose a new collision model for porous
dust aggregates on the basis of our N-body experiments on aggregate collisions.
We first obtain empirical data on porosity changes between the classical limits
of ballistic cluster-cluster and particle-cluster aggregation. Using the data,
we construct a recipe for the porosity change due to general hit-and-stick
collisions as well as formulae for the aerodynamical and collisional cross
sections. Simple coagulation simulations using the extended Smoluchowski method
show that our collision model explains the fractal dimensions of porous
aggregates observed in a full N-body simulation and a laboratory experiment.
Besides, we discover that aggregates at the high-mass end of the distribution
can have a considerably small aerodynamical cross section per unit mass
compared with aggregates of lower masses. We point out an important implication
of this discovery for dust growth in protoplanetary disks.Comment: 17 pages, 15 figures; v2: version to appear in ApJ (typos corrected
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
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier
The sticking of micron sized dust particles due to surface forces in
circumstellar disks is the first stage in the production of asteroids and
planets. The key ingredients that drive this process are the relative velocity
between the dust particles in this environment and the complex physics of dust
aggregate collisions. Here we present the results of a collision model, which
is based on laboratory experiments of these aggregates. We investigate the
maximum aggregate size and mass that can be reached by coagulation in
protoplanetary disks. We model the growth of dust aggregates at 1 AU at the
midplane at three different gas densities. We find that the evolution of the
dust does not follow the previously assumed growth-fragmentation cycles.
Catastrophic fragmentation hardly occurs in the three disk models. Furthermore
we see long lived, quasi-steady states in the distribution function of the
aggregates due to bouncing. We explore how the mass and the porosity change
upon varying the turbulence parameter and by varying the critical mass ratio of
dust particles. Particles reach Stokes numbers of roughly 10^-4 during the
simulations. The particle growth is stopped by bouncing rather than
fragmentation in these models. The final Stokes number of the aggregates is
rather insensitive to the variations of the gas density and the strength of
turbulence. The maximum mass of the particles is limited to approximately 1
gram (chondrule-sized particles). Planetesimal formation can proceed via the
turbulent concentration of these aerodynamically size-sorted chondrule-sized
particles.Comment: accepted for publication in A&
Decimetre dust aggregates in protoplanetary discs
The growth of planetesimals is an essential step in planet formation.
Decimetre-size dust agglomerates mark a transition point in this growth
process. In laboratory experiments we simulated the formation, evolution, and
properties of decimetre-scale dusty bodies in protoplanetary discs. Small
sub-mm size dust aggregates consisting of micron-size SiO particles
randomly interacted with dust targets of varying initial conditions in a
continuous sequence of independent collisions. Impact velocities were 7.7 m/s
on average and in the range expected for collisions with decimetre bodies in
protoplanetary discs. The targets all evolved by forming dust \emph{crusts}
with up to several cm thickness and a unique filling factor of 31% 3%. A
part of the projectiles sticks directly. In addition, some projectile fragments
slowly return to the target by gravity. All initially porous parts of the
surface, i.e. built from the slowly returning fragments, are compacted and
firmly attached to the underlying dust layers by the subsequent impacts. Growth
is possible at impact angles from 0 (central collision) to
70. No growth occurs at steeper dust surfaces. We measured the
velocity, angle, and size distribution of collision fragments. The average
restitution coefficient is 3.8% or 0.29 m/s ejection velocity. Ejecta sizes are
comparable to the projectile sizes. The high filling factor is close to the
most compact configuration of dust aggregates by local compression (%). This implies that the history of the surface formation and target growth
is completely erased. In view of this, the filling factor of 31% seems to be a
universal value in the collision experiments of all self-consistently evolving
targets at the given impact velocities. We suggest that decimetre and probably
larger bodies can simply be characterised by one single filling factor.Comment: 10 pages, 9 figure
Opacity of fluffy dust aggregates
Context. Dust grains coagulate to form dust aggregates in protoplanetary
disks. Their porosity can be extremely high in the disks. Although disk
emission may come from fluffy dust aggregates, the emission has been modeled
with compact grains. Aims. We aim to reveal the mass opacity of fluffy
aggregates from infrared to millimeter wavelengths with the filling factor
ranging from 1 down to . Methods. We use Mie calculations with an
effective medium theory. The monomers are assumed to be 0.1 sized
grains, which is much shorter than the wavelengths that we focus on. Results.
We find that the absorption mass opacity of fluffy aggregates are characterized
by the product , where is the dust radius and is the filling
factor, except for the interference structure. The scattering mass opacity is
also characterized by at short wavelengths while it is higher in more
fluffy aggregates at long wavelengths. We also derive the analytic formula of
the mass opacity and find that it reproduces the Mie calculations. We also
calculate the expected difference of the emission between compact and fluffy
aggregates in protoplanetary disks with a simple dust growth and drift model.
We find that compact grains and fluffy aggregates can be distinguished by the
radial distribution of the opacity index . The previous observation of
the radial distribution of is consistent with the fluffy case, but more
observations are required to distinguish between fluffy or compact. In
addition, we find that the scattered light would be another way to distinguish
between compact grains and fluffy aggregates.Comment: 16 pages, 17 figures, published in A&A, 568, A4
Dust coagulation in protoplanetary disks: porosity matters
Context: Sticking of colliding dust particles through van der Waals forces is
the first stage in the grain growth process in protoplanetary disks, eventually
leading to the formation of comets, asteroids and planets. A key aspect of the
collisional evolution is the coupling between dust and gas motions, which
depends on the internal structure (porosity) of aggregates. Aims: To quantify
the importance of the internal structure on the collisional evolution of
particles, and to create a new coagulation model to investigate the difference
between porous and compact coagulation in the context of a turbulent
protoplanetary disk. Methods: We have developed simple prescriptions for the
collisional evolution of porosity of grain-aggregates in grain-grain
collisions. Three regimes can then be distinguished: `hit-and-stick' at low
velocities, with an increase in porosity; compaction at intermediate
velocities, with a decrease of porosity; and fragmentation at high velocities.
(..) Results: (..) We can discern three different stages in the particle growth
process (..) We find that when compared to standard, compact models of
coagulation, porous growth delays the onset of settling, because the surface
area-to-mass ratio is higher, a consequence of the build-up of porosity during
the initial stages. As a result, particles grow orders of magnitudes larger in
mass before they rain-out to the mid-plane. Depending on the turbulent
viscosity and on the position in the nebula, aggregates can grow to (porous)
sizes of ~ 10 cm in a few thousand years. We also find that collisional
energies are higher than in the limited PCA/CCA fractal models, thereby
allowing aggregates to restructure. It is concluded that the microphysics of
collisions plays a key role in the growth process.Comment: 21 pages, 15 figures. Accepted for publication in A&A. Abstract
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