1,425 research outputs found
Origin of the slow dynamics and the aging of a soft glass
We study by light microscopy a soft glass consisting of a compact arrangement
of polydisperse elastic spheres. We show that its slow and non-stationary
dynamics results from the unavoidable small fluctuations of temperature, which
induce intermittent local mechanical shear in the sample, because of thermal
expansion and contraction. Temperature-induced shear provokes both reversible
and irreversible rearrangements whose amplitude decreases with time, leading to
an exponential slowing down of the dynamics with sample age.Comment: published in PRL 97, 238301, 200
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
Bounded Languages Meet Cellular Automata with Sparse Communication
Cellular automata are one-dimensional arrays of interconnected interacting
finite automata. We investigate one of the weakest classes, the real-time
one-way cellular automata, and impose an additional restriction on their
inter-cell communication by bounding the number of allowed uses of the links
between cells. Moreover, we consider the devices as acceptors for bounded
languages in order to explore the borderline at which non-trivial decidability
problems of cellular automata classes become decidable. It is shown that even
devices with drastically reduced communication, that is, each two neighboring
cells may communicate only constantly often, accept bounded languages that are
not semilinear. If the number of communications is at least logarithmic in the
length of the input, several problems are undecidable. The same result is
obtained for classes where the total number of communications during a
computation is linearly bounded
A Study of Brain Networks Associated with Swallowing Using Graph-Theoretical Approaches
Functional connectivity between brain regions during swallowing tasks is still not well understood. Understanding these complex interactions is of great interest from both a scientific and a clinical perspective. In this study, functional magnetic resonance imaging (fMRI) was utilized to study brain functional networks during voluntary saliva swallowing in twenty-two adult healthy subjects (all females, 23.1±1.52 years of age). To construct these functional connections, we computed mean partial correlation matrices over ninety brain regions for each participant. Two regions were determined to be functionally connected if their correlation was above a certain threshold. These correlation matrices were then analyzed using graph-theoretical approaches. In particular, we considered several network measures for the whole brain and for swallowing-related brain regions. The results have shown that significant pairwise functional connections were, mostly, either local and intra-hemispheric or symmetrically inter-hemispheric. Furthermore, we showed that all human brain functional network, although varying in some degree, had typical small-world properties as compared to regular networks and random networks. These properties allow information transfer within the network at a relatively high efficiency. Swallowing-related brain regions also had higher values for some of the network measures in comparison to when these measures were calculated for the whole brain. The current results warrant further investigation of graph-theoretical approaches as a potential tool for understanding the neural basis of dysphagia. © 2013 Luan et al
The Physics of the Colloidal Glass Transition
As one increases the concentration of a colloidal suspension, the system
exhibits a dramatic increase in viscosity. Structurally, the system resembles a
liquid, yet motions within the suspension are slow enough that it can be
considered essentially frozen. This kinetic arrest is the colloidal glass
transition. For several decades, colloids have served as a valuable model
system for understanding the glass transition in molecular systems. The spatial
and temporal scales involved allow these systems to be studied by a wide
variety of experimental techniques. The focus of this review is the current
state of understanding of the colloidal glass transition. A brief introduction
is given to important experimental techniques used to study the glass
transition in colloids. We describe features of colloidal systems near and in
glassy states, including tremendous increases in viscosity and relaxation
times, dynamical heterogeneity, and ageing, among others. We also compare and
contrast the glass transition in colloids to that in molecular liquids. Other
glassy systems are briefly discussed, as well as recently developed synthesis
techniques that will keep these systems rich with interesting physics for years
to come.Comment: 56 pages, 18 figures, Revie
Exponential Random Graph Modeling for Complex Brain Networks
Exponential random graph models (ERGMs), also known as p* models, have been
utilized extensively in the social science literature to study complex networks
and how their global structure depends on underlying structural components.
However, the literature on their use in biological networks (especially brain
networks) has remained sparse. Descriptive models based on a specific feature
of the graph (clustering coefficient, degree distribution, etc.) have dominated
connectivity research in neuroscience. Corresponding generative models have
been developed to reproduce one of these features. However, the complexity
inherent in whole-brain network data necessitates the development and use of
tools that allow the systematic exploration of several features simultaneously
and how they interact to form the global network architecture. ERGMs provide a
statistically principled approach to the assessment of how a set of interacting
local brain network features gives rise to the global structure. We illustrate
the utility of ERGMs for modeling, analyzing, and simulating complex
whole-brain networks with network data from normal subjects. We also provide a
foundation for the selection of important local features through the
implementation and assessment of three selection approaches: a traditional
p-value based backward selection approach, an information criterion approach
(AIC), and a graphical goodness of fit (GOF) approach. The graphical GOF
approach serves as the best method given the scientific interest in being able
to capture and reproduce the structure of fitted brain networks
An inner warp discovered in the disk around HD 110058 using VLT/SPHERE and HST/STIS
An edge-on debris disk was detected in 2015 around the young, nearby A0V star
HD 110058. The disk showed features resembling those seen in the disk of beta
Pictoris that could indicate the presence of a perturbing planetary-mass
companion in the system. We investigated new and archival scattered light
images of the disk in order to characterise its morphology and spectrum. In
particular, we analysed the disk's warp to constrain the properties of possible
planetary perturbers. Our work uses data from two VLT/SPHERE observations and
archival data from HST/STIS. We measured the morphology of the disk by
analysing vertical profiles along the length of the disk to extract the
centroid spine position and vertical height. We extracted the surface
brightness and reflectance spectrum of the disk. We detect the disk between 20
au (with SPHERE) and 150 au (with STIS), at a position angle of
159.60.6. Analysis of the spine shows an asymmetry between
the two sides of the disk, with a 3.40.9 warp between ~20 au
and 60 au. The disk is marginally vertically resolved in scattered light, with
a vertical aspect ratio of 9.30.7% at 45 au. The extracted reflectance
spectrum is featureless, flat between 0.95 micron and 1.1 micron, and red from
1.1 micron to 1.65 micron. The outer parts of the disk are also asymmetric with
a tilt between the two sides compatible with a disk made of forward-scattering
particles and seen not perfectly edge-on, suggesting an inclination of
<84. The presence of an undetected planetary-mass companion on an
inclined orbit with respect to the disk could explain the warp. The
misalignment of the inner parts of the disk with respect to the outer disk
suggests a warp that has not yet propagated to the outer parts of the disk,
favouring the scenario of an inner perturber as the origin of the warp.Comment: 17 pages, 15 figures, 3 tables; accepted for publication in A&
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