56 research outputs found
Size-selective concentration of chondrules and other small particles in protoplanetary nebula turbulence
Size-selective concentration of particles in a weakly turbulent
protoplanetary nebula may be responsible for the initial collection of
chondrules and other constituents into primitive body precursors. This paper
presents the main elements of this process of turbulent concentration. In the
terrestrial planet region, both the characteristic size and size distribution
of chondrules are explained. "Fluffier" particles would be concentrated in
nebula regions which were at a lower gas density and/or more intensely
turbulent. The spatial distribution of concentrated particle density obeys
multifractal scaling}, suggesting a close tie to the turbulent cascade process.
This scaling behavior allows predictions of the probability distributions for
concentration in the protoplanetary nebula to be made. Large concentration
factors (>10^5) are readily obtained, implying that numerous zones of particle
density significantly exceeding the gas density could exist. If most of the
available solids were actually in chondrule sized particles, the ensuing
particle mass density would become so large that the feedback effects on gas
turbulence due to mass loading could no longer be neglected. This paper
describes the process, presenting its basic elements and some implications,
without including the effects of mass loading.Comment: 34 pages, 7 figures; in press for Astrophys. J; expected Jan 01 2001
issu
Gravitational structure formation in scale relativity
In the framework of the theory of scale relativity, we suggest a solution to
the cosmological problem of the formation and evolution of gravitational
structures on many scales. This approach is based on the giving up of the
hypothesis of differentiability of space-time coordinates. As a consequence of
this generalization, space-time is not only curved, but also fractal. In
analogy with Einstein's general relativistic methods, we describe the effects
of space fractality on motion by the construction of a covariant derivative.
The principle of equivalence allows us to write the equation of dynamics as a
geodesics equation that takes the form of the equation of free Galilean motion.
Then, after a change of variables, this equation can be integrated in terms of
a gravitational Schrodinger equation that involves a new fundamental
gravitational coupling constant, alpha_{g} = w_{0}/c. Its solutions give
probability densities that quantitatively describe precise morphologies in the
position space and in the velocity space. Finally the theoretical predictions
are successfully checked by a comparison with observational data: we find that
matter is self-organized in accordance with the solutions of the gravitational
Schrodinger equation on the basis of the universal constant w_{0}=144.7 +- 0.7
km/s (and its multiples and sub-multiples), from the scale of our Earth and the
Solar System to large scale structures of the UniverseComment: 34 pages, 42 figures. Higher quality figures adde
The Evolution of the Water Distribution in a Viscous Protoplanetary Disk
(Abridged) Astronomical observations have shown that protoplanetary disks are
dynamic objects through which mass is transported and accreted by the central
star. Age dating of meteorite constituents shows that their creation,
evolution, and accumulation occupied several Myr, and over this time disk
properties would evolve significantly. Moreover, on this timescale, solid
particles decouple from the gas in the disk and their evolution follows a
different path. Here we present a model which tracks how the distribution of
water changes in an evolving disk as the water-bearing species experience
condensation, accretion, transport, collisional destruction, and vaporization.
Because solids are transported in a disk at different rates depending on their
sizes, the motions will lead to water being concentrated in some regions of a
disk and depleted in others. These enhancements and depletions are consistent
with the conditions needed to explain some aspects of the chemistry of
chondritic meteorites and formation of giant planets. The levels of
concentration and depletion, as well as their locations, depend strongly on the
combined effects of the gaseous disk evolution, the formation of rapidly
migrating rubble, and the growth of immobile planetesimals. We present examples
of evolution under a range of plausible assumptions and demonstrate how the
chemical evolution of the inner region of a protoplanetary disk is intimately
connected to the physical processes which occur in the outer regions.Comment: 45 pages, 7 figures, revised for publication in Icaru
On the nature, formation and diversity of particulate coherent structures in microgravity conditions and their relevance to materials science and problems of astrophysical interest
Different phenomena related to the spontaneous accumulation of solid particles dispersed in a fluid medium in microgravity conditions are discussed, with an emphasis on recent discoveries and potential links with the general field of astrophysical fluid-dynamics on the one hand, and with terrestrial applications in the field of materials science on the other hand. With special attention to the typical physical forces at play in such an environment, namely, surface-tension gradients, oscillatory residual gravity components, inertial disturbances and forces of an electrostatic nature, specific experimental and numerical examples are presented to provide inputs for an increased understanding of the underlying cause-and-effect relationships. Studying these systems can be seen as a matter of understanding how macroscopic scenarios arise from the cooperative behaviour of sub-parts or competing mechanisms (nonlinearities and interdependencies on various spatial and temporal scales). Through a critical assessment of the properties displayed by the resulting structures (which appear in the form of one-dimensional circuits formed by aligned particles, planar accumulation surfaces, three-dimensional compact structures resembling “quadrics”, micro-crystallites or fractal aggregates), we discuss a possible classification of the related particle attractors in the space of parameters according to the prevailing effect
Selective Aggregation Experiments on Planetesimal Formation and Mercury-Like Planets
Much of a planet's composition could be determined right at the onset of
formation. Laboratory experiments can constrain these early steps. This
includes static tensile strength measurements or collisions carried out under
Earth's gravity and on various microgravity platforms. Among the variety of
extrasolar planets which eventually form are (Exo)-Mercury, terrestrial planets
with high density. If they form in inner protoplanetary disks, high temperature
experiments are mandatory but they are still rare. Beyond the initial process
of hit-and-stick collisions, some additional selective processing might be
needed to explain Mercury. In analogy to icy worlds, such planets might, e.g.,
form in environments which are enriched in iron. This requires methods to
separate iron and silicate at early stages. Photophoresis might be one viable
way. Mercury and Mercury-like planets might also form due to the ferromagnetic
properties of iron and mechanisms like magnetic aggregation in disk magnetic
fields might become important. This review highlights some of the mechanisms
with the potential to trigger Mercury formation.Comment: This article belongs to the Special Issue of Geosciences: Detection
and Characterization of Extrasolar Planet
Microgravity Particle Research on the Space Station
Science questions that could be addressed by a Space Station Microgravity Particle Research Facility for studying small suspended particles were discussed. Characteristics of such a facility were determined. Disciplines covered include astrophysics and the solar nebula, planetary science, atmospheric science, exobiology and life science, and physics and chemistry
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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