453 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
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments
The growth processes from protoplanetary dust to planetesimals are not fully
understood. Laboratory experiments and theoretical models have shown that
collisions among the dust aggregates can lead to sticking, bouncing, and
fragmentation. However, no systematic study on the collisional outcome of
protoplanetary dust has been performed so far so that a physical model of the
dust evolution in protoplanetary disks is still missing. We intend to map the
parameter space for the collisional interaction of arbitrarily porous dust
aggregates. This parameter space encompasses the dust-aggregate masses, their
porosities and the collision velocity. With such a complete mapping of the
collisional outcomes of protoplanetary dust aggregates, it will be possible to
follow the collisional evolution of dust in a protoplanetary disk environment.
We use literature data, perform own laboratory experiments, and apply simple
physical models to get a complete picture of the collisional interaction of
protoplanetary dust aggregates. In our study, we found four different types of
sticking, two types of bouncing, and three types of fragmentation as possible
outcomes in collisions among protoplanetary dust aggregates. We distinguish
between eight combinations of porosity and mass ratio. For each of these cases,
we present a complete collision model for dust-aggregate masses between 10^-12
and 10^2 g and collision velocities in the range 10^-4 to 10^4 cm/s for
arbitrary porosities. This model comprises the collisional outcome, the
mass(es) of the resulting aggregate(s) and their porosities. We present the
first complete collision model for protoplanetary dust. This collision model
can be used for the determination of the dust-growth rate in protoplanetary
disks.Comment: accepted by Astronomy and Astrophysic
Numerical Simulations of Highly Porous Dust Aggregates in the Low-Velocity Collision Regime
A highly favoured mechanism of planetesimal formation is collisional growth.
Single dust grains, which follow gas flows in the protoplanetary disc, hit each
other, stick due to van der Waals forces and form fluffy aggregates up to
centimetre size. The mechanism of further growth is unclear since the outcome
of aggregate collisions in the relevant velocity and size regime cannot be
investigated in the laboratory under protoplanetary disc conditions. Realistic
statistics of the result of dust aggregate collisions beyond decimetre size is
missing for a deeper understanding of planetary growth. Joining experimental
and numerical efforts we want to calibrate and validate a computer program that
is capable of a correct simulation of the macroscopic behaviour of highly
porous dust aggregates. After testing its numerical limitations thoroughly we
will check the program especially for a realistic reproduction of various
benchmark experiments. We adopt the smooth particle hydrodynamics (SPH)
numerical scheme with extensions for the simulation of solid bodies and a
modified version of the Sirono porosity model. Experimentally measured
macroscopic material properties of silica dust are implemented. We calibrate
and test for the compressive strength relation and the bulk modulus. SPH has
already proven to be a suitable tool to simulate collisions at rather high
velocities. In this work we demonstrate that its area of application can not
only be extended to low-velocity experiments and collisions. It can also be
used to simulate the behaviour of highly porous objects in this velocity regime
to a very high accuracy.The result of the calibration process in this work is
an SPH code that can be utilised to investigate the collisional outcome of
porous dust in the low-velocity regime.Comment: accepted by Astronomy & Astrophysic
Collisions of inhomogeneous pre-planetesimals
In the framework of the coagulation scenario, kilometre-sized planetesimals
form by subsequent collisions of pre-planetesimals of sizes from centimetre to
hundreds of metres. Pre-planetesimals are fluffy, porous dust aggregates, which
are inhomogeneous owing to their collisional history. Planetesimal growth can
be prevented by catastrophic disruption in pre-planetesimal collisions above
the destruction velocity threshold. We develop an inhomogeneity model based on
the density distribution of dust aggregates, which is assumed to be a Gaussian
distribution with a well-defined standard deviation. As a second input
parameter, we consider the typical size of an inhomogeneous clump. These input
parameters are easily accessible by laboratory experiments. For the simulation
of the dust aggregates, we utilise a smoothed particle hydrodynamics (SPH) code
with extensions for modelling porous solid bodies. The porosity model was
previously calibrated for the simulation of silica dust, which commonly serves
as an analogue for pre-planetesimal material. The inhomogeneity is imposed as
an initial condition on the SPH particle distribution. We carry out collisions
of centimetre-sized dust aggregates of intermediate porosity. We vary the
standard deviation of the inhomogeneous distribution at fixed typical clump
size. The collision outcome is categorised according to the four-population
model. We show that inhomogeneous pre-planetesimals are more prone to
destruction than homogeneous aggregates. Even slight inhomogeneities can lower
the threshold for catastrophic disruption. For a fixed collision velocity, the
sizes of the fragments decrease with increasing inhomogeneity.
Pre-planetesimals with an active collisional history tend to be weaker. This is
a possible obstacle to collisional growth and needs to be taken into account in
future studies of the coagulation scenario.Comment: 12 pages, 9 figures, 4 table
Frequency Shifts and Linewidth Changes of Infrared-Active Phonons in Double-Layered High-Temperature Superconductors
We calculate frequency shifts and changes in linewidths of infrared-active
phonons within a shell model for the bare phononic system coupled to an
electronic double-layer structure with inter-layer charge transfer. The
theoretical concept is applied to YBaCuO yielding a good description of
experimental results in the normal state as well as at the transition to the
superconducting state.Comment: 8 pages, LaTex, SISSA-CM-93-00
The four-populations model: a new classification scheme for pre-planetesimal collisions
Within the collision growth scenario for planetesimal formation, the growth
step from centimetre sized pre-planetesimals to kilometre sized planetesimals
is still unclear. The formation of larger objects from the highly porous
pre-planetesimals may be halted by a combination of fragmentation in disruptive
collisions and mutual rebound with compaction. However, the right amount of
fragmentation is necessary to explain the observed dust features in late T
Tauri discs. Therefore, detailed data on the outcome of pre-planetesimal
collisions is required and has to be presented in a suitable and precise
format. We propose and apply a new classification scheme for pre-planetesimal
collisions based on the quantitative aspects of four fragment populations: the
largest and second largest fragment, a power-law population, and a
sub-resolution population. For the simulations of pre-planetesimal collisions,
we adopt the SPH numerical scheme with extensions for the simulation of porous
solid bodies. By means of laboratory benchmark experiments, this model was
previously calibrated and tested for the correct simulation of the compaction,
bouncing, and fragmentation behaviour of macroscopic highly porous silica dust
aggregates. It is shown that previous attempts to map collision data were much
too oriented on qualitatively categorising into sticking, bouncing, and
fragmentation events. We show that the four-populations model encompasses all
previous categorisations and in addition allows for transitions. This is
because it is based on quantitative characteristic attributes of each
population such as the mass, kinetic energy, and filling factor. As a
demonstration of the applicability and the power of the four-populations model,
we utilise it to present the results of a study on the influence of collision
velocity in head-on collisions of intermediate porosity aggregates.Comment: 14 pages, 11 figures, 5 tables, to be published in Astronomy and
Astrophysic
\'Free Collisions in a Microgravity Many-Particle Experiment. II. The Collision Dynamics of Dust-Coated Chondrules
The formation of planetesimals in the early Solar System is hardly
understood, and in particular the growth of dust aggregates above millimeter
sizes has recently turned out to be a difficult task in our understanding [Zsom
et al. 2010, A&A, 513, A57]. Laboratory experiments have shown that dust
aggregates of these sizes stick to one another only at unreasonably low
velocities. However, in the protoplanetary disk, millimeter-sized particles are
known to have been ubiquitous. One can find relics of them in the form of solid
chondrules as the main constituent of chondrites. Most of these chondrules were
found to feature a fine-grained rim, which is hypothesized to have formed from
accreting dust grains in the solar nebula. To study the influence of these
dust-coated chondrules on the formation of chondrites and possibly
planetesimals, we conducted collision experiments between millimeter-sized,
dust-coated chondrule analogs at velocities of a few cm/s. For 2 and 3 mm
diameter chondrule analogs covered by dusty rims of a volume filling factor of
0.18 and 0.35-0.58, we found sticking velocities of a few cm/s. This velocity
is higher than the sticking velocity of dust aggregates of the same size. We
therefore conclude that chondrules may be an important step towards a deeper
understanding of the collisional growth of larger bodies. Moreover, we analyzed
the collision behavior in an ensemble of dust aggregates and non-coated
chondrule analogs. While neither the dust aggregates nor the solid chondrule
analogs show sticking in collisions among their species, we found an enhanced
sicking efficiency in collisions between the two constituents, which leads us
to the conjecture that chondrules might act as "catalyzers" for the growth of
larger bodies in the young Solar System
Levodopa‐induced dyskinesia are mediated by cortical gamma oscillations in experimental Parkinsonism
Background Levodopa is the most efficacious drug in the symptomatic therapy of motor symptoms in Parkinson's disease (PD); however, long‐term treatment is often complicated by troublesome levodopa‐induced dyskinesia (LID). Recent evidence suggests that LID might be related to increased cortical gamma oscillations. Objective The objective of this study was to test the hypothesis that cortical high‐gamma network activity relates to LID in the 6‐hydroxydopamine model and to identify new biomarkers for adaptive deep brain stimulation (DBS) therapy in PD. Methods We recorded and analyzed primary motor cortex (M1) electrocorticogram data and motor behavior in freely moving 6‐OHDA lesioned rats before and during a daily treatment with levodopa for 3 weeks. The results were correlated with the abnormal involuntary movement score (AIMS) and used for generalized linear modeling (GLM). Results Levodopa reverted motor impairment, suppressed beta activity, and, with repeated administration, led to a progressive enhancement of LID. Concurrently, we observed a highly significant stepwise amplitude increase in finely tuned gamma (FTG) activity and gamma centroid frequency. Whereas AIMS and FTG reached their maximum after the 4th injection and remained on a stable plateau thereafter, the centroid frequency of the FTG power continued to increase thereafter. Among the analyzed gamma activity parameters, the fraction of longest gamma bursts showed the strongest correlation with AIMS. Using a GLM, it was possible to accurately predict AIMS from cortical recordings. Conclusions FTG activity is tightly linked to LID and should be studied as a biomarker for adaptive DBS
The Physics of Protoplanetesimal Dust Agglomerates. IV. Towards a Dynamical Collision Model
Recent years have shown many advances in our knowledge of the collisional
evolution of protoplanetary dust. Based on a variety of dust-collision
experiments in the laboratory, our view of the growth of dust aggregates in
protoplanetary disks is now supported by a deeper understanding of the physics
involved in the interaction between dust agglomerates. However, the parameter
space, which determines the collisional outcome, is huge and sometimes
inaccessible to laboratory experiments. Very large or fluffy dust aggregates
and extremely low collision velocities are beyond the boundary of today's
laboratories. It is therefore desirable to augment our empirical knowledge of
dust-collision physics with a numerical method to treat arbitrary aggregate
sizes, porosities and collision velocities. In this article, we implement
experimentally-determined material parameters of highly porous dust aggregates
into a Smooth Particle Hydrodynamics (SPH) code, in particular an
omnidirectional compressive-strength and a tensile-strength relation. We also
give a prescription of calibrating the SPH code with compression and
low-velocity impact experiments. In the process of calibration, we developed a
dynamic compressive-strength relation and estimated a relation for the shear
strength. Finally, we defined and performed a series of benchmark tests and
found the agreement between experimental results and numerical simulations to
be very satisfactory. SPH codes have been used in the past to study collisions
at rather high velocities. At the end of this work, we show examples of future
applications in the low-velocity regime of collisional evolution.Comment: accepted by The astrophysical Journa
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