523 research outputs found
Debris and micrometeorite impact measurements in the laboratory
A method was developed to simulate space debris in the laboratory. This method, which is an outgrowth of research in inertial confinement fusion (ICF), uses laser ablation to accelerate material. Using this method, single 60 micron aluminum spheres were accelerated to 15 km/sec and larger 500 micron aluminum spheres were accelerated to 2 km/sec. Also, many small (less than 10 micron diameter) irregularly shaped particles were accelerated to speeds of 100 km/sec
Integrable atomtronic interferometry
High sensitivity quantum interferometry requires more than just access to
entangled states. It is achieved through deep understanding of quantum
correlations in a system. Integrable models offer the framework to develop this
understanding. We communicate the design of interferometric protocols for an
integrable model that describes the interaction of bosons in a four-site
configuration. Analytic formulae for the quantum dynamics of certain
observables are computed. These expose the system's functionality as both an
interferometric identifier, and producer, of NOON states. Being equivalent to a
controlled-phase gate acting on two hybrid qudits, this system also highlights
an equivalence between Heisenberg-limited interferometry and quantum
information. These results are expected to open new avenues for
integrability-enhanced atomtronic technologies.Comment: 6 pages, 4 figures, 1 tabl
The Cosmic Infrared Background at 1.25 microns and 2.2 microns using DIRBE and 2MASS: a contribution not due to galaxies ?
Using the 2MASS 2nd Incremental Data Release and the Zodiacal-Subtracted
Mission Average maps of COBE/DIRBE, we estimate the cosmic background in the J
(1.25 micron) and K (2.2 microns) bands using selected areas representing 550
square degrees of sky. We find a J background of 22.9 \pm 7.0 kJy/sr (54.0 \pm
16.8 nW/m2/sr) and a K background of 20.4 \pm 4.9 kJy/sr (27.8 \pm 6.7
nW/m2/sr). This large scale study shows that the main uncertainty comes from
the residual zodiacal emission. The cosmic background we obtain is
significantly higher than integrated galaxy counts (3.6 \pm 0.8 kJy/sr and 5.3
\pm 1.2 kJy/sr for J and K, respectively), suggesting either an increase of the
galaxy luminosity function for magnitudes fainter than 30 or the existence of
another contribution to the cosmic background from primeval stars, black holes,
or relic particle decay.Comment: 20 pages, 6 figures, accepted in Ap
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: IV. Cosmological Implications
In this paper we examine the cosmological constraints of the recent DIRBE and
FIRAS detection of the extragalactic background light between 125-5000 microns
on the metal and star formation histories of the universe.Comment: 38 pages and 9 figures. Accepted for publications in The
Astrophysical Journa
Free induction signal from biexcitons and bound excitons
A theory of the free induction signal from biexcitons and bound excitons is
presented. The simultaneous existence of the exciton continuum and a bound
state is shown to result in a new type of time dependence of the free
induction. The optically detected signal increases in time and oscillates with
increasing amplitude until damped by radiative or dephasing processes.
Radiative decay is anomalously fast and can result in strong picosecond pulses.
The expanding area of a coherent exciton polarization (inflating antenna),
produced by the exciting pulse, is the underlying physical mechanism. The
developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1
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Combined transcriptomic-(1)H NMR metabonomic study reveals yhat monoethylhexyl phthalate stimulates adipogenesis and glyceroneogenesis in human adipocytes
Adipose tissue is a major storage site for lipophilic environmental contaminants. The environmental metabolic disruptor hypothesis postulates that some pollutants can promote obesity or metabolic disorders by activating nuclear receptors involved in the control of energetic homeostasis. In this context, monoethylhexyl phthalate (MEHP) is of particular concern since it was shown to activate the peroxisome proliferator-activated receptor Ī³ (PPARĪ³) in 3T3-L1 murine preadipocytes. In the present work, we used an untargeted, combined transcriptomic-(1)H NMR-based metabonomic approach to describe the overall effect of MEHP on primary cultures of human subcutaneous adipocytes differentiated in vitro. MEHP stimulated rapidly and selectively the expression of genes involved in glyceroneogenesis, enhanced the expression of the cytosolic phosphoenolpyruvate carboxykinase, and reduced fatty acid release. These results demonstrate that MEHP increased glyceroneogenesis and fatty acid reesterification in human adipocytes. A longer treatment with MEHP induced the expression of genes involved in triglycerides uptake, synthesis, and storage; decreased intracellular lactate, glutamine, and other amino acids; increased aspartate and NAD, and resulted in a global increase in triglycerides. Altogether, these results indicate that MEHP promoted the differentiation of human preadipocytes to adipocytes. These mechanisms might contribute to the suspected obesogenic effect of MEHP
Well posedness of an isothermal diffusive model for binary mixtures of incompressible fluids
We consider a model describing the behavior of a mixture of two
incompressible fluids with the same density in isothermal conditions. The model
consists of three balance equations: continuity equation, Navier-Stokes
equation for the mean velocity of the mixture, and diffusion equation
(Cahn-Hilliard equation). We assume that the chemical potential depends upon
the velocity of the mixture in such a way that an increase of the velocity
improves the miscibility of the mixture. We examine the thermodynamic
consistence of the model which leads to the introduction of an additional
constitutive force in the motion equation. Then, we prove existence and
uniqueness of the solution of the resulting differential problem
Weak pairwise correlations imply strongly correlated network states in a neural population
Biological networks have so many possible states that exhaustive sampling is
impossible. Successful analysis thus depends on simplifying hypotheses, but
experiments on many systems hint that complicated, higher order interactions
among large groups of elements play an important role. In the vertebrate
retina, we show that weak correlations between pairs of neurons coexist with
strongly collective behavior in the responses of ten or more neurons.
Surprisingly, we find that this collective behavior is described quantitatively
by models that capture the observed pairwise correlations but assume no higher
order interactions. These maximum entropy models are equivalent to Ising
models, and predict that larger networks are completely dominated by
correlation effects. This suggests that the neural code has associative or
error-correcting properties, and we provide preliminary evidence for such
behavior. As a first test for the generality of these ideas, we show that
similar results are obtained from networks of cultured cortical neurons.Comment: Full account of work presented at the conference on Computational and
Systems Neuroscience (COSYNE), 17-20 March 2005, in Salt Lake City, Utah
(http://cosyne.org
A Fluorescent Aerogel for Capture and Identification of Interplanetary and Interstellar Dust
Contemporary interstellar dust has never been analyzed in the laboratory,
despite its obvious astronomical importance and its potential as a probe of
stellar nucleosynthesis and galactic chemical evolution. Here we report the
discovery of a novel fluorescent aerogel which is capable of capturing
hypervelocity dust grains and passively recording their kinetic energies. An
array of these "calorimetric" aerogel collectors in low earth orbit would lead
to the capture and identification of large numbers of interstellar dust grains.Comment: 13 pages, 4 figures, to appear in The Astrophysical Journa
Multiple Deeply Divergent Denisovan Ancestries in Papuans
Genome sequences are known for two archaic
homininsāNeanderthals and Denisovansāwhich
interbred with anatomically modern humans as
they dispersed out of Africa. We identified high-confidence
archaic haplotypes in 161 new genomes
spanning 14 island groups in Island Southeast
Asia and New Guinea and found large stretches of
DNA that are inconsistent with a single introgressing
Denisovan origin. Instead, modern Papuans carry
hundreds of gene variants from two deeply divergent
Denisovan lineages that separated over 350
thousand years ago. Spatial and temporal structure
among these lineages suggest that introgression
from one of these Denisovan groups predominantly
took place east of the Wallace line and continued
until near the end of the Pleistocene. A third Denisovan
lineage occurs in modern East Asians. This
regional mosaic suggests considerable complexity
in archaic contact, with modern humans interbreeding
with multiple Denisovan groups that were
geographically isolated from each other over deep
evolutionary time
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