449 research outputs found
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
Fish in the city
Aquaculture is the most recent addition to animal husbandry and it is the fastest growing food production industry. Its contribution to world food security in the 21st century is already significant and it is bound to continue to grow because demand for fish for human consumption is rapidly increasing whereas fish supplies from ocean fisheries are likely to decline. The rapid evolution of aquaculture involved a host of innovations of which many were based on R&D activities by public and private research organizations. Applied R&D tends to be the more effective the better focused it is on specific research problems or opportunities. Among the many possible aquaculture production systems on which aquaculture R&D might focus are recirculation aquaculture systems and in this paper we explore crucial aspects of the potential of urban recirculation aquaculture. Our exploration begins with a vision of recirculation aquaculture production plants located at the fringes of cities of converging economies. Such production systems are distinctly different from conventional urban aquaculture systems based on urban sewage. We scrutinize our vision from four perspectives: (i) the expected demand for aquaculture fish from urban consumers; (ii) cost competitiveness of fish produced at the fringes of cities as compared to fish produced in the rural hinterland; (iii) the potential for integration of urban recirculation aquaculture production into the modern food supply chains that are now emerging in converging economies, and (iv) the ecological footprint of aquaculture production compared to that of chicken production. Based on trends in the growth of urban populations world-wide and trends in demand for fish for food we estimate a total urban demand for aquaculture finfish between 11 and 51 million tons in 2025. We use von ThĂŒnen's location theory to provide support for the vision to locate recirculation aquaculture plants not within cities and not in their rural hinterland but on the fringes of cities. Moreover, we argue that tightly controlled recirculation aquaculture production would seem to be particularly well suited for being integrated into modern food supply chains. Finally, we compare the ecological footprint of recirculation aquaculture fish with that of industrially produced chicken and we find that the ecological balance depends on the source of energy used. We conclude our exploratory study with some thoughts on the implication for aquaculture R&D of the potential for recirculation aquaculture located on the fringes of cities in emerging economy countries. --
Who's in Control? Proficiency and L1 Influence on L2 Processing
Abstract
We report three reaction time (RT)/event-related brain potential (ERP) semantic priming lexical decision experiments that explore the following in relation to L1 activation during L2 processing: (1) the role of L2 proficiency, (2) the role of sentence context, and (3) the locus of L1 activations (ortho-graphic vs. semantic). All experiments used German (L1) homonyms translated into English (L2) to form prime-target pairs (pine-jaw for Kiefer) to test whether the L1 caused interference in an all-L2 experiment. Both RTs and ERPs were measured on targets. Experiment 1 revealed reversed priming in the N200 component and RTs for low-proficiency learners, but only RT interference for high-proficiency participants. Experiment 2 showed that once the words were processed in sentence context, the low-proficiency participants still showed reversed N200 and RT priming, whereas the high-proficiency group showed no effects. Experiment 3 tested native English speakers with the words in sentence context and showed a null result comparable to the high-proficiency group. Based on these results, we argue that cognitive control relating to translational activation is modulated by (1) L2 proficiency, as the early interference in the N200 was observed only for low-proficiency learners, and (2) sentence context, as it helps high-proficiency learners control L1 activation. As reversed priming was observed in the N200 and not the N400 component, we argue that (3) the locus of the L1 activations was orthographic. Implications in terms of bilingual word recognition and the functional role of the N200 ERP component are discussed.</jats:p
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&
Fish in the city
Aquaculture is the most recent addition to animal husbandry and it is the fastest growing food production industry. Its contribution to world food security in the 21st century is already significant and it is bound to continue to grow because demand for fish for human consumption is rapidly increasing whereas fish supplies from ocean fisheries are likely to decline. The rapid evolution of aquaculture involved a host of innovations of which many were based on R&D activities by public and private research organizations. Applied R&D tends to be the more effective the better focused it is on specific research problems or opportunities. Among the many possible aquaculture production systems on which aquaculture R&D might focus are recirculation aquaculture systems and in this paper we explore crucial aspects of the potential of urban recirculation aquaculture. Our exploration begins with a vision of recirculation aquaculture production plants located at the fringes of cities of converging economies. Such production systems are distinctly different from conventional urban aquaculture systems based on urban sewage. We scrutinize our vision from four perspectives: (i) the expected demand for aquaculture fish from urban consumers; (ii) cost competitiveness of fish produced at the fringes of cities as compared to fish produced in the rural hinterland; (iii) the potential for integration of urban recirculation aquaculture production into the modern food supply chains that are now emerging in converging economies, and (iv) the ecological footprint of aquaculture production compared to that of chicken production. Based on trends in the growth of urban populations world-wide and trends in demand for fish for food we estimate a total urban demand for aquaculture finfish between 11 and 51 million tons in 2025. We use von ThĂŒnen's location theory to provide support for the vision to locate recirculation aquaculture plants not within cities and not in their rural hinterland but on the fringes of cities. Moreover, we argue that tightly controlled recirculation aquaculture production would seem to be particularly well suited for being integrated into modern food supply chains. Finally, we compare the ecological footprint of recirculation aquaculture fish with that of industrially produced chicken and we find that the ecological balance depends on the source of energy used. We conclude our exploratory study with some thoughts on the implication for aquaculture R&D of the potential for recirculation aquaculture located on the fringes of cities in emerging economy countries
Levodopaâinduced 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, longâterm treatment is often complicated by troublesome levodopaâinduced 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 highâgamma network activity relates to LID in the 6âhydroxydopamine 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 6âOHDA lesioned rats before and during a daily treatment with levodopa for 3âweeks. 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
Accelerated physical emulation of Bayesian inference in spiking neural networks
The massively parallel nature of biological information processing plays an
important role for its superiority to human-engineered computing devices. In
particular, it may hold the key to overcoming the von Neumann bottleneck that
limits contemporary computer architectures. Physical-model neuromorphic devices
seek to replicate not only this inherent parallelism, but also aspects of its
microscopic dynamics in analog circuits emulating neurons and synapses.
However, these machines require network models that are not only adept at
solving particular tasks, but that can also cope with the inherent
imperfections of analog substrates. We present a spiking network model that
performs Bayesian inference through sampling on the BrainScaleS neuromorphic
platform, where we use it for generative and discriminative computations on
visual data. By illustrating its functionality on this platform, we implicitly
demonstrate its robustness to various substrate-specific distortive effects, as
well as its accelerated capability for computation. These results showcase the
advantages of brain-inspired physical computation and provide important
building blocks for large-scale neuromorphic applications.Comment: This preprint has been published 2019 November 14. Please cite as:
Kungl A. F. et al. (2019) Accelerated Physical Emulation of Bayesian
Inference in Spiking Neural Networks. Front. Neurosci. 13:1201. doi:
10.3389/fnins.2019.0120
The first stages of planet formation in binary systems: How far can dust coagulation proceed?
We examine the first phase of the core accretion model, namely the dust
growth/fragmentation in binary systems. In our model, a gas and dust disk is
present around the primary star and is perturbed by the secondary. We study the
effects of a secondary with/without eccentricity on the dust population to
determine what sizes the aggregates can reach and how that compares to the dust
population in disks around single stars.
We find that the secondary star has two effects on the dust population. 1.)
The disk is truncated due to the presence of the secondary star and the maximum
mass of the particles is decreased in the lowered gas densities. This effect is
dominant in the outer disk. 2.) The perturbation of the secondary pumps up the
eccentricity of the gas disk, which in turn increases the relative velocity
between the dust and the gas. Therefore the maximum particle sizes are further
decreased. The second effect of the secondary influences the entire disk.
Coagulation is efficiently reduced even at the very inner parts of the disk.
The average mass of the particles is reduced by four orders of magnitude (as a
consequence, the stopping time is reduced by one order of magnitude) in disks
around binary systems compared to dust in disks around single stars.Comment: accepted for publication in A&
The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier
The sticking of micron sized dust particles due to surface forces in
circumstellar disks is the first stage in the production of asteroids and
planets. The key ingredients that drive this process are the relative velocity
between the dust particles in this environment and the complex physics of dust
aggregate collisions. Here we present the results of a collision model, which
is based on laboratory experiments of these aggregates. We investigate the
maximum aggregate size and mass that can be reached by coagulation in
protoplanetary disks. We model the growth of dust aggregates at 1 AU at the
midplane at three different gas densities. We find that the evolution of the
dust does not follow the previously assumed growth-fragmentation cycles.
Catastrophic fragmentation hardly occurs in the three disk models. Furthermore
we see long lived, quasi-steady states in the distribution function of the
aggregates due to bouncing. We explore how the mass and the porosity change
upon varying the turbulence parameter and by varying the critical mass ratio of
dust particles. Particles reach Stokes numbers of roughly 10^-4 during the
simulations. The particle growth is stopped by bouncing rather than
fragmentation in these models. The final Stokes number of the aggregates is
rather insensitive to the variations of the gas density and the strength of
turbulence. The maximum mass of the particles is limited to approximately 1
gram (chondrule-sized particles). Planetesimal formation can proceed via the
turbulent concentration of these aerodynamically size-sorted chondrule-sized
particles.Comment: accepted for publication in A&
Pattern representation and recognition with accelerated analog neuromorphic systems
Despite being originally inspired by the central nervous system, artificial
neural networks have diverged from their biological archetypes as they have
been remodeled to fit particular tasks. In this paper, we review several
possibilites to reverse map these architectures to biologically more realistic
spiking networks with the aim of emulating them on fast, low-power neuromorphic
hardware. Since many of these devices employ analog components, which cannot be
perfectly controlled, finding ways to compensate for the resulting effects
represents a key challenge. Here, we discuss three different strategies to
address this problem: the addition of auxiliary network components for
stabilizing activity, the utilization of inherently robust architectures and a
training method for hardware-emulated networks that functions without perfect
knowledge of the system's dynamics and parameters. For all three scenarios, we
corroborate our theoretical considerations with experimental results on
accelerated analog neuromorphic platforms.Comment: accepted at ISCAS 201
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