604 research outputs found
Erosion of dust aggregates
Aims: The aim of this work is to gain a deeper insight into how much
different aggregate types are affected by erosion. Especially, it is important
to study the influence of the velocity of the impacting projectiles. We also
want to provide models for dust growth in protoplanetary disks with simple
recipes to account for erosion effects.
Methods: To study the erosion of dust aggregates we employed a molecular
dynamics approach that features a detailed micro-physical model of the
interaction of spherical grains. For the first time, the model has been
extended by introducing a new visco-elastic damping force which requires a
proper calibration. Afterwards, different sample generation methods were used
to cover a wide range of aggregate types.
Results: The visco-elastic damping force introduced in this work turns out to
be crucial to reproduce results obtained from laboratory experiments. After
proper calibration, we find that erosion occurs for impact velocities of 5 m/s
and above. Though fractal aggregates as formed during the first growth phase
are most susceptible to erosion, we observe erosion of aggregates with rather
compact surfaces as well.
Conclusions: We find that bombarding a larger target aggregate with small
projectiles results in erosion for impact velocities as low as a few m/s. More
compact aggregates suffer less from erosion. With increasing projectile size
the transition from accretion to erosion is shifted to higher velocities. This
allows larger bodies to grow through high velocity collisions with smaller
aggregates.Comment: accepted for publication in Astronomy & Astrophysic
Tensile & shear strength of porous dust agglomerates
Context.Within the sequential accretion scenario of planet formation, planets
are build up through a sequence sticking collisions. The outcome of collisions
between porous dust aggregates is very important for the growth from very small
dust particles to planetesimals. In this work we determine the necessary
material properties of dust aggregates as a function the porosity.
Aims: Continuum models such as SPH that are capable of simulating collisions
of macroscopic dust aggregates require a set of material parameters. Some of
them such as the tensile and shear strength are difficult to obtain from
laboratory experiments. The aim of this work is to determine these parameters
from ab-initio molecular dynamics simulations.
Methods: We simulate the behavior of porous dust aggregates using a detailed
micro-physical model of the interaction of spherical grains that includes
adhesion forces, rolling, twisting, and sliding. Using different methods of
preparing the samples we study the strength behavior of our samples with
varying porosity and coordination number of the material.
Results: For the tensile strength, we can reproduce data from laboratory
experiments very well. For the shear strength, there are no experimental data
available. The results from our simulations differ significantly from previous
theoretical models, which indicates that the latter might not be sufficient to
describe porous dust aggregates.
Conclusions: We have provided functional behavior of tensile and shear
strength of porous dust aggregates as a function of the porosity that can be
directly applied in continuum simulations of these objects in planet formation
scenarios.Comment: Accepted for publication in A&
The physics of protoplanetesimal dust agglomerates. VII The low-velocity collision behavior of large dust agglomerates
We performed micro-gravity collision experiments in our laboratory drop-tower
using 5-cm-sized dust agglomerates with volume filling factors of 0.3 and 0.4,
respectively. This work is an extension of our previous experiments reported in
Beitz et al. (2011) to aggregates of more than one order of magnitude higher
masses. The dust aggregates consisted of micrometer-sized silica particles and
were macroscopically homogeneous. We measured the coefficient of restitution
for collision velocities ranging from 1 cm/s to 0.5 m/s, and determined the
fragmentation velocity. For low velocities, the coefficient of restitution
decreases with increasing impact velocity, in contrast to findings by Beitz et
al. (2011). At higher velocities, the value of the coefficient of restitution
becomes constant, before the aggregates break at the onset of fragmentation. We
interpret the qualitative change in the coefficient of restitution as the
transition from a solid-body-dominated to a granular-medium-dominated behavior.
We complement our experiments by molecular dynamics simulations of porous
aggregates and obtain a reasonable match to the experimental data. We discuss
the importance of our experiments for protoplanetary disks, debris disks, and
planetary rings. The work is an extensional study to previous work of our group
and gives a new insight in the velocity dependency of the coefficient of
restitution due to improved measurements, better statistics and a theoretical
approach
Memoria und Konfession. Süddeutsche Grabdenkmäler im Zeitalter der Konfessionalisierung
Wie wirkt sich der theologische Wandel in nachreformatorischer Zeit auf die Gestaltung und die Funktion von Grabdenkmälern aus? Wurden Grabdenkmäler bewusst für die Darstellung persönlicher Glaubensvorstellungen und die Vermittlung konfessionsspezifischer Botschaften genutzt? Inwieweit lässt sich eine konfessionsübergreifende Kontinuität spätmittelalterlicher Traditionen belegen? Die Grundlage der vorliegenden Arbeit bildet eine empirische Untersuchung spätmittelalterlicher und frühneuzeitlicher Grabdenkmäler aus vier süddeutschen Territorien (Baden-Baden, Württemberg, Hohenlohe, Bayern). Im Fokus steht die Analyse einer Vielzahl von Gestaltungselementen, die einen direkten Bezug zur Heiligen Schrift herstellen (Bibelzitate, biblische Szenen, Segensformeln und Attribute)
Compression Behaviour of Porous Dust Agglomerates
The early planetesimal growth proceeds through a sequence of sticking
collisions of dust agglomerates. Very uncertain is still the relative velocity
regime in which growth rather than destruction can take place. The outcome of a
collision depends on the bulk properties of the porous dust agglomerates.
Continuum models of dust agglomerates require a set of material parameters that
are often difficult to obtain from laboratory experiments. Here, we aim at
determining those parameters from ab-initio molecular dynamics simulations. Our
goal is to improveon the existing model that describe the interaction of
individual monomers. We use a molecular dynamics approach featuring a detailed
micro-physical model of the interaction of spherical grains. The model includes
normal forces, rolling, twisting and sliding between the dust grains. We
present a new treatment of wall-particle interaction that allows us to perform
customized simulations that directly correspond to laboratory experiments. We
find that the existing interaction model by Dominik & Tielens leads to a too
soft compressive strength behavior for uni and omni-directional compression.
Upon making the rolling and sliding coefficients stiffer we find excellent
agreement in both cases. Additionally, we find that the compressive strength
curve depends on the velocity with which the sample is compressed. The modified
interaction strengths between two individual dust grains will lead to a
different behaviour of the whole dust agglomerate. This will influences the
sticking probabilities and hence the growth of planetesimals. The new parameter
set might possibly lead to an enhanced sticking as more energy can be stored in
the system before breakup.Comment: 11 pages, 14 figures, accepted for publication in A&
Collisions of small ice particles under microgravity conditions - II. Does the chemical composition of the ice change the collisional properties?
Context. Understanding the collisional properties of ice is important for understanding both the early stages of planet formation and the evolution of planetary ring systems. Simple chemicals such as methanol and formic acid are known to be present in cold protostellar regions alongside the dominant water ice; they are also likely to be incorporated into planets which form in protoplanetary disks, and planetary ring systems. However, the effect of the chemical composition of the ice on its collisional properties has not yet been studied.Aims. Collisions of 1.5 cm ice spheres composed of pure crystalline water ice, water with 5% methanol, and water with 5% formic acid were investigated to determine the effect of the ice composition on the collisional outcomes.Methods. The collisions were conducted in a dedicated experimental instrument, operated under microgravity conditions, at relative particle impact velocities between 0.01 and 0.19 ms-1, temperatures between 131 and 160 K and a pressure of around 10-5Results. A range of coefficients of restitution were found, with no correlation between this and the chemical composition, relative impact velocity, or temperature.Conclusions. We conclude that the chemical composition of the ice (at the level of 95% water ice and 5% methanol or formic acid) does not affect the collisional properties at these temperatures and pressures due to the inability of surface wetting to take place. At a level of 5% methanol or formic acid, the structure is likely to be dominated by crystalline water ice, leading to no change in collisional properties. The surface roughness of the particles is the dominant factor in explaining the range of coefficients of restitution
Progress towards identifying the neurofibromatosis (NF1) gene
Von Recklinghausen neurofibromatosis (NF1) is a common autosomal dominant disorder of humans. Linkage analysis has recently mapped the NF1 gene to the proximal long arm of chromosome 17. The identification of two NF1 patients with balanced translocations has now allowed the location of the gene to be narrowed to a few hundred kilobases of chromosome band 17q11.2, using a combination of somatic cell hybrid technology, linking clones and pulsed field gel electrophoresis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28154/1/0000606.pd
Genetic flanking markers refine diagnostic criteria and provide insights into the genetics of Von Hippel Lindau disease.
Molecular analysis of a myxoid chondrosarcoma with rearrangements of chromosomes 10 and 22
Inhibitory pathways and the inhibition of luteinizing hormone-releasing hormone release by alcohol
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