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

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

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)

Towards Image Ambient Lighting Normalization

Lighting normalization is a crucial but underexplored restoration task with broad applications. However, existing works often simplify this task within the context of shadow removal, limiting the light sources to one and oversimplifying the scene, thus excluding complex self-shadows and restricting surface classes to smooth ones. Although promising, such simplifications hinder generalizability to more realistic settings encountered in daily use. In this paper, we propose a new challenging task termed Ambient Lighting Normalization (ALN), which enables the study of interactions between shadows, unifying image restoration and shadow removal in a broader context. To address the lack of appropriate datasets for ALN, we introduce the large-scale high-resolution dataset Ambient6K, comprising samples obtained from multiple light sources and including self-shadows resulting from complex geometries, which is the first of its kind. For benchmarking, we select various mainstream methods and rigorously evaluate them on Ambient6K. Additionally, we propose IFBlend, a novel strong baseline that maximizes Image-Frequency joint entropy to selectively restore local areas under different lighting conditions, without relying on shadow localization priors. Experiments show that IFBlend achieves SOTA scores on Ambient6K and exhibits competitive performance on conventional shadow removal benchmarks compared to shadow-specific models with mask priors. The dataset, benchmark, and code are available at https://github.com/fvasluianu97/IFBlend

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

Improved Detection of Germline Mutations in Korean VHL Patients by Multiple Ligation-dependent Probe Amplification Analysis

von Hippel-Lindau (VHL) disease is an autosomal dominant inherited tumor syndrome characterized by the development of tumors in the eye, brain, spinal cord, inner ear, adrenal gland, pancreas, kidney, and epididymis, associated with germline mutations in the VHL gene. We used sequentially sequencing method and multiple ligation-dependent probe amplification (MLPA) analysis and detected germline mutations in the VHL in 15/15 (100%) of VHL patients fulfilling the clinical criteria. Of the 15 distinct mutations detected, large deletions were detected in 5/15 (33.3%) patients, including 4/15 (26.7%) partial deletions and 1/15 (6.6%) deletion of the entire VHL gene by MLPA and the remainder were point mutations detected by sequencing method, of which five mutations were novel. Using MLPA analysis, we detected large deletions including both partial deletions and complete gene deletion, which has not been reported in Korean VHL patients. In conclusion, sequential application of sequencing method and MLPA analysis might make possible to identify germline mutations in most patients with VHL