244 research outputs found
Low-velocity collisions of centimeter-sized dust aggregates
Collisions between centimeter- to decimeter-sized dusty bodies are important
to understand the mechanisms leading to the formation of planetesimals. We thus
performed laboratory experiments to study the collisional behavior of dust
aggregates in this size range at velocities below and around the fragmentation
threshold. We developed two independent experimental setups with the same goal
to study the effects of bouncing, fragmentation, and mass transfer in free
particle-particle collisions. The first setup is an evacuated drop tower with a
free-fall height of 1.5 m, providing us with 0.56 s of microgravity time so
that we observed collisions with velocities between 8 mm/s and 2 m/s. The
second setup is designed to study the effect of partial fragmentation (when
only one of the two aggregates is destroyed) and mass transfer in more detail.
It allows for the measurement of the accretion efficiency as the samples are
safely recovered after the encounter. Our results are that for very low
velocities we found bouncing as could be expected while the fragmentation
velocity of 20 cm/s was significantly lower than expected. We present the
critical energy for disruptive collisions Q*, which showed up to be at least
two orders of magnitude lower than previous experiments in the literature. In
the wide range between bouncing and disruptive collisions, only one of the
samples fragmented in the encounter while the other gained mass. The accretion
efficiency in the order of a few percent of the particle's mass is depending on
the impact velocity and the sample porosity. Our results will have consequences
for dust evolution models in protoplanetary disks as well as for the strength
of large, porous planetesimal bodies
Recapitulation of selective nuclear import and export with a perfectly repeated 12mer GLFG peptide
The permeability barrier of nuclear pore complexes (NPCs) controls nucleocytoplasmic transport. It retains inert macromolecules while allowing facilitated passage of importins and exportins, which in turn shuttle cargo into or out of cell nuclei. The barrier can be described as a condensed phase assembled from cohesive FG repeat domains. NPCs contain several distinct FG domains, each comprising variable repeats. Nevertheless, we now found that sequence heterogeneity is no fundamental requirement for barrier function. Instead, we succeeded in engineering a perfectly repeated 12mer GLFG peptide that self-assembles into a barrier of exquisite transport selectivity and fast transport kinetics. This barrier recapitulates RanGTPase-controlled importin- and exportin-mediated cargo transport and thus represents an ultimately simplified experimental model system. An alternative proline-free sequence forms an amyloid FG phase. Finally, we discovered that FG phases stain bright with âDNA-specificâ DAPI/ Hoechst probes, and that such dyes allow for a photo-induced block of nuclear transport
Breaking through: The effects of a velocity distribution on barriers to dust growth
It is unknown how far dust growth can proceed by coagulation. Obstacles to
collisional growth are the fragmentation and bouncing barriers. However, in all
previous simulations of the dust-size evolution in protoplanetary disks, only
the mean collision velocity has been considered, neglecting that a small but
possibly important fraction of the collisions will occur at both much lower and
higher velocities. We study the effect of the probability distribution of
impact velocities on the collisional dust growth barriers. Assuming a
Maxwellian velocity distribution for colliding particles to determine the
fraction of sticking, bouncing, and fragmentation, we implement this in a
dust-size evolution code. We also calculate the probability of growing through
the barriers and the growth timescale in these regimes. We find that the
collisional growth barriers are not as sharp as previously thought. With the
existence of low-velocity collisions, a small fraction of the particles manage
to grow to masses orders of magnitude above the main population. A particle
velocity distribution softens the fragmentation barrier and removes the
bouncing barrier. It broadens the size distribution in a natural way, allowing
the largest particles to become the first seeds that initiate sweep-up growth
towards planetesimal sizes.Comment: 4 pages, 3 figures. Accepted for publication as a Letter in Astronomy
and Astrophysic
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Collisions of mm-size dust aggregates play a crucial role in the early phases
of planet formation. We developed a laboratory setup to observe collisions of
dust aggregates levitating at mbar pressures and elevated temperatures of 800
K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm
in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller
than 25 \mum in size. We find that for all impact energies in the studied range
sticking occurs at a probability of 32.1 \pm 2.5% on average. In general, the
sticking probability decreases with increasing impact parameter. The sticking
probability increases with energy density (impact energy per contact area). We
also observe collisions of aggregates that were formed by a previous sticking
of two larger aggregates. Partners of these aggregates can be detached by a
second collision with a probability of on average 19.8 \pm 4.0%. The measured
accretion efficiencies are remarkably high compared to other experimental
results. We attribute this to the rel. large dust grains used in our
experiments, which make aggregates more susceptible to restructuring and energy
dissipation. Collisional hardening by compaction might not occur as the
aggregates are already very compact with only 54 \pm 1% porosity. The
disassembly of previously grown aggregates in collisions might stall further
aggregate growth. However, owing to the levitation technique and the limited
data statistics, no conclusive statement about this aspect can yet be given. We
find that the detachment efficiency decreases with increasing velocities and
accretion dominates in the higher velocity range. For high accretion
efficiencies, our experiments suggest that continued growth in the mm-range
with larger constituent grains would be a viable way to produce larger
aggregates, which might in turn form the seeds to proceed to growing
planetesimals.Comment: 9 pages, 20 figure
Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits
Context. Grains in circumstellar disks are believed to grow by mutual
collisions and subsequent sticking due to surface forces. Results of many
fields of research involving circumstellar disks, such as radiative transfer
calculations, disk chemistry, magneto-hydrodynamic simulations largely depend
on the unknown grain size distribution.
Aims. As detailed calculations of grain growth and fragmentation are both
numerically challenging and computationally expensive, we aim to find simple
recipes and analytical solutions for the grain size distribution in
circumstellar disks for a scenario in which grain growth is limited by
fragmentation and radial drift can be neglected.
Methods. We generalize previous analytical work on self-similar steady-state
grain distributions. Numerical simulations are carried out to identify under
which conditions the grain size distributions can be understood in terms of a
combination of power-law distributions. A physically motivated fitting formula
for grain size distributions is derived using our analytical predictions and
numerical simulations.
Results. We find good agreement between analytical results and numerical
solutions of the Smoluchowski equation for simple shapes of the kernel
function. The results for more complicated and realistic cases can be fitted
with a physically motivated "black box" recipe presented in this paper. Our
results show that the shape of the dust distribution is mostly dominated by the
gas surface density (not the dust-to-gas ratio), the turbulence strength and
the temperature and does not obey an MRN type distribution.Comment: 16 pages, 9 figures, accepted for publication in A&
Ring shaped dust accumulation in transition disks
Context.Transition disks are believed to be the final stages of
protoplanetary disks, during which a forming planetary system or
photoevaporation processes open a gap in the inner disk, drastically changing
the disk structure. From theoretical arguments it is expected that dust growth,
fragmentation and radial drift are strongly influenced by gas disk structure,
and pressure bumps in disks have been suggested as key features that may allow
grains to converge and grow efficiently.
Aims. We want to study how the presence of a large planet in a disk
influences the growth and radial distribution of dust grains, and how
observable properties are linked to the mass of the planet.
Methods. We combine two-dimensional hydrodynamical disk simulations of
disk-planet interactions with state-of-the-art coagulation/fragmentation models
to simulate the evolution of dust in a disk which has a gap created by a
massive planet. We compute images at different wavelengths and illustrate our
results using the example of the transition disk LkCa15.
Results. The gap opened by a planet and the long-range interaction between
the planet and the outer disk create a single large pressure bump outside the
planetary orbit. Millimeter-sized particles form and accumulate at the pressure
maximum and naturally produce ring-shaped sub-millimeter emission that is
long-lived because radial drift no longer depletes the large grain population
of the disk. For large planet masses around 9 , the pressure
maximum and, therefore, the ring of millimeter particles is located at
distances that can be more than twice the star-planet separation, creating a
large spatial separation between the gas inner edge of the outer disk and the
peak millimeter emission. Smaller grains do get closer to the gap and we
predict how the surface brightness varies at different wavelengths.Comment: Accepted for publication in Astronomy and Astrophysic
A simple model for the evolution of the dust population in protoplanetary disks
Context: The global size and spatial distribution of dust is an important
ingredient in the structure and evolution of protoplanetary disks and in the
formation of larger bodies, such as planetesimals. Aims: We aim to derive
simple equations that explain the global evolution of the dust surface density
profile and the upper limit of the grain size distribution and which can
readily be used for further modeling or for interpreting of observational data.
Methods: We have developed a simple model that follows the upper end of the
dust size distribution and the evolution of the dust surface density profile.
This model is calibrated with state-of-the-art simulations of dust evolution,
which treat dust growth, fragmentation, and transport in viscously evolving gas
disks. Results: We find very good agreement between the full dust-evolution
code and the toy model presented in this paper. We derive analytical profiles
that describe the dust-to-gas ratios and the dust surface density profiles well
in protoplanetary disks, as well as the radial flux by solid material "rain
out", which is crucial for triggering any gravity assisted formation of
planetesimals. We show that fragmentation is the dominating effect in the inner
regions of the disk leading to a dust surface density exponent of -1.5, while
the outer regions at later times can become drift-dominated, yielding a dust
surface density exponent of -0.75. Our results show that radial drift is not
efficient in fragmenting dust grains. This supports the theory that small dust
grains are resupplied by fragmentation due to the turbulent state of the disk.Comment: 12 pages, 10 figures, accepted to A&
Thermal history modeling of the H chondrite parent body
The cooling histories of individual meteorites can be empirically
reconstructed by using ages from different radioisotopic chronometers with
distinct closure temperatures. For a group of meteorites derived from a single
parent body such data permit the reconstruction of the cooling history and
properties of that body. Particularly suited are H chondrites because precise
radiometric ages over a wide range of closure temperatures are available. A
thermal evolution model for the H chondrite parent body is constructed by using
all H chondrites for which at least three different radiometric ages are
available. Several key parameters determining the thermal evolution of the H
chondrite parent body and the unknown burial depths of the H chondrites are
varied until an optimal fit is obtained. The fit is performed by an 'evolution
algorithm'. Empirical data for eight samples are used for which radiometric
ages are available for at least three different closure temperatures. A set of
parameters for the H chondrite parent body is found that yields excellent
agreement (within error bounds) between the thermal evolution model and
empirical data of six of the examined eight chondrites. The new thermal model
constrains the radius and formation time of the H chondrite parent body
(possibly (6) Hebe), the initial burial depths of the individual H chondrites,
the average surface temperature of the body, the average initial porosity of
the material the body accreted from, and the initial 60Fe content of the H
chondrite parent body.Comment: 16 pages, 7 figure
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