9,664 research outputs found
Endografts, Pressure, and the Abdominal Aortic Aneurysm
Abdominal aortic aneurysms (AAA) are an expansion in diameter of the
abdominal aorta and their rupture is a leading cause of mortality. One of the treatments
for AAA is the implantation of an endograft (also called a stent graft), a combination of
fabric and metal stents, to provide a new conduit for blood and shield the aneurysm sac
from direct pressurization. After implantation of the stent graft, the aneurysm may
shrink, grow, or stabilize in diameter ? even in the absence of apparent flow into the sac
? in some cases resulting in graft failure through component separation, kinking, or loss
of seal at its ends.
Greater understanding of AAA and treated AAA could provide insight on how
treatment might be modified to improve treatment methods and/or design devices to be
more effective in a wider range of patients. Computational models provide a means to
investigate the biomechanics of endografts treating AAA through analysis of the
endografts, the AAA, and the combination of them.
Axisymmetric models of endograft-treated AAA showed that peak von Mises
stress within the wall varied between 533 kPa and 1200 kPa when different material
properties for the endograft were used. The patient-specific models, built from time series of patient CT scans with similar patient history but different outcomes, show that
wall shrinkage and stability can be related to the level of stresses within the vessel wall,
with the shrinking AAA showing a greater reduction by endograft treatment and a lower
final value of average von Mises stress. The reduction in pressure felt by the wall is
local to the central sac region. The inclusion of thrombus is also essential to accurate
stress estimation.
The combination of axisymmetric and patient-specific computational models
explains in further detail the biomechanics of endograft treatment. The patient-specific
reconstruction models show that when effectively deployed and reducing the pressure
felt in the AAA wall, the graft is under tension in the sac region and compression at its
ends
Switching dynamics of surface stabilized ferroelectric liquid crystal cells: effects of anchoring energy asymmetry
We study both theoretically and experimentally switching dynamics in surface
stabilized ferroelectric liquid crystal cells with asymmetric boundary
conditions. In these cells the bounding surfaces are treated differently to
produce asymmetry in their anchoring properties. Our electro-optic measurements
of the switching voltage thresholds that are determined by the peaks of the
reversal polarization current reveal the frequency dependent shift of the
hysteresis loop. We examine the predictions of the uniform dynamical model with
the anchoring energy taken into account. It is found that the asymmetry effects
are dominated by the polar contribution to the anchoring energy. Frequency
dependence of the voltage thresholds is studied by analyzing the properties of
time-periodic solutions to the dynamical equation (cycles). For this purpose,
we apply the method that uses the parameterized half-period mappings for the
approximate model and relate the cycles to the fixed points of the composition
of two half-period mappings. The cycles are found to be unstable and can only
be formed when the driving frequency is lower than its critical value. The
polar anchoring parameter is estimated by making a comparison between the
results of modelling and the experimental data for the shift vs frequency
curve. For a double-well potential considered as a deformation of the
Rapini-Papoular potential, the branch of stable cycles emerges in the low
frequency region separated by the gap from the high frequency interval for
unstable cycles.Comment: 35 pages, 15 figure
Liquid Crystal-Solid Interface Structure at the Antiferroelectric-Ferroelectric Phase Transition
Total Internal Reflection (TIR) is used to probe the molecular organization
at the surface of a tilted chiral smectic liquid crystal at temperatures in the
vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are
interpreted using an exact analytical solution of a real model for
ferroelectric order at the surface. In the mixture T3, ferroelectric surface
order is expelled with the bulk ferroelectric-antiferroelectric transition. The
conditions for ferroelectric order at the surface of an antiferroelectric bulk
are presented
Dirac eigenvalues for a softcore Coulomb potential in d dimensions
A single fermion is bound by a softcore central Coulomb potential V(r) =
-v/(r^q + b^q)^(1/q), v>0, b>0, q \ge 1, in d>1 spatial dimensions. Envelope
theory is used to construct analytic lower bounds for the discrete Dirac energy
spectrum. The results are compared to accurate eigenvalues obtained
numerically.Comment: 8 pages 1 figur
Discovery of a young and massive stellar cluster: Spectrophotometric near-infrared study of Masgomas-1
Context: Recent near-infrared data have contributed to the discovery of new
(obscured) massive stellar clusters and massive stellar populations in
previously known clusters in our Galaxy. These discoveries lead us to view the
Milky Way as an active star-forming machine.
Aims: The main purpose of this work is to determine physically the main
parameters (distance, size, total mass and age) of Masgomas-1, the first
massive cluster discovered by our systematic search programme.
Methods: Using near-infrared (J, H, and Ks) photometry we selected 23 OB-type
and five red supergiant candidates for multi-object H- and K-spectroscopy and
spectral classification.
Results: Of the 28 spectroscopically observed stars, 17 were classified as
OB-type, four as supergiants, one as an A-type dwarf star, and six as late-type
giant stars. The presence of a supergiant population implies a massive nature
of Masgomas-1, supported by our estimate of the cluster initial total mass of
(1.94\pm0.28)\cdot10^4 M_{sun}, obtained after integrating of the cluster mass
function. The distance estimate of 3.53 kpc locates the cluster closer than the
Scutum--Centaurus base but still within that Galactic arm. The presence of an
O9V star and red supergiants in the same population indicates that the cluster
age is in the range of 8 to 10 Myr.Comment: 11 pages, 6 figures, 2 tables, A&A accepte
Extended longevity of DNA preservation in Levantine Paleolithic sediments, Sefunim Cave, Israel
Paleogenomic research can elucidate the evolutionary history of human and faunal populations. Although the Levant is a key land-bridge between Africa and Eurasia, thus far, relatively little ancient DNA data has been generated from this region, since DNA degrades faster in warm climates. As sediments can be a source of ancient DNA, we analyzed 33 sediment samples from different sedimentological contexts in the Paleolithic layers of Sefunim Cave (Israel). Four contained traces of ancient Cervidae and Hyaenidae mitochondrial DNA. Dating by optical luminescence and radiocarbon indicates that the DNA comes from layers between 30,000 and 70,000 years old, surpassing theoretical expectations regarding the longevity of DNA deposited in such a warm environment. Both identified taxa are present in the zooarchaeological record of the site but have since gone extinct from the region, and a geoarchaeological study suggests little movement of the sediments after their deposition, lending further support to our findings. We provide details on the local conditions in the cave, which we hypothesize were particularly conducive to the long-term preservation of DNA—information that will be pertinent for future endeavors aimed at recovering ancient DNA from the Levant and other similarly challenging contexts
Role of heavy-meson exchange in pion production near threshold
Recent calculations of -wave pion production have severely underestimated
the accurately known \ total cross section near
threshold. In these calculations, only the single-nucleon axial-charge operator
is considered. We have calculated, in addition to the one-body term, the
two-body contributions to this reaction that arise from the exchange of mesons.
We find that the inclusion of the scalar -meson exchange current (and
lesser contributions from other mesons) increases the cross section by about a
factor of five, and leads to excellent agreement with the data. The results are
neither very sensitive to changes in the distorting potential that generates
the wave function, nor to different choices for the meson-nucleon form
factors. We argue that \ data provide direct
experimental evidence for meson-exchange contributions to the axial current.Comment: 28 Pages, IU-NTC #93-0
Near-infrared spectroscopy in NGC 7538
The characterisation of the stellar population toward young high-mass
star-forming regions allows to constrain fundamental cluster properties like
distance and age. These are essential when using high-mass clusters as probes
to conduct Galactic studies. NGC 7538 is a star-forming region with an embedded
stellar population only unearthed in the near-infrared. We present the first
near-infrared spectro-photometric study of the candidate high-mass stellar
content in NGC 7538. We obtained H and K spectra of 21 sources with both the
multi-object and long-slit modes of LIRIS at the WHT, and complement these data
with sub-arcsecond JHKs photometry of the region using the imaging mode of the
same instrument. We find a wide variety of objects within the studied stellar
population of NGC 7538. Our results discriminate between a stellar population
associated to the HII region, but not contained within its extent, and several
pockets of more recent star formation. We report the detection of CO bandhead
emission toward several sources as well as other features indicative of a young
stellar nature. We infer a spectro-photometric distance of 2.7+-0.5 kpc, an age
spread in the range 0.5-2.2 Myr and a total mass ~1.7x10^3 Msun for the older
population.Comment: 11 pages, 8 figures, 1 table, accepted by A&
Dynamic, adaptive changes in MAO-A binding after alterations in substrate availability: an in vivo [11C]-harmine positron emission tomography study
Monoamine oxidase A (MAO-A) is an important target in the pathophysiology and therapeutics of major depressive disorder, aggression, and neurodegenerative conditions. We measured the effect of changes in MAO-A substrate on MAO-A binding in regions implicated in affective and neurodegenerative disease with [11C]-harmine positron emission tomography in healthy volunteers. Monoamine oxidase A VT, an index of MAO-A density, was decreased (mean: 14%±9%) following tryptophan depletion in prefrontal cortex (P<0.031), and elevated (mean: 17%±11%) in striatum following carbidopa–levodopa administration (P<0.007). These findings suggest an adaptive role for MAO-A in maintaining monoamine neurotransmitter homeostasis by rapidly compensating fluctuating monoamine levels
A multi-wavelength census of star formation activity in the young embedded cluster around Serpens/G3-G6
Aims. The aim of this paper is to characterise the star formation activity in
the poorly studied embedded cluster Serpens/G3-G6, located ~ 45' (3 pc) to the
south of the Serpens Cloud Core, and to determine the luminosity and mass
functions of its population of Young Stellar Objects (YSOs).
Methods. Multi-wavelength broadband photometry was obtained to sample the
near and mid-IR spectral energy distributions to separate YSOs from field stars
and classify the YSO evolutionary stage. ISOCAM mapping in the two filters LW2
(5-8.5 um) and LW3 (12-18 um) of a 19' x 16' field was combined with JHKs data
from 2MASS, Ks data from Arnica/NOT, and L' data from SIRCA/NOT. Continuum
emission at 1.3 mm (IRAM) and 3.6 cm (VLA) was mapped to study the cloud
structure and the coldest/youngest sources. Deep narrow band imaging at the
2.12 um S(1) line of H2 from NOTCam/NOT was obtained to search for signs of
bipolar outflows.
Results. We have strong evidence for a stellar population of 31 Class II
sources, 5 flat-spectrum sources, 5 Class I sources, and two Class 0 sources.
Our method does not sample the Class III sources. The cloud is composed of two
main dense clumps aligned along a ridge over ~ 0.5 pc plus a starless core
coinciding with absorption features seen in the ISOCAM maps. We find two
S-shaped bipolar collimated flows embedded in the NE clump, and propose the two
driving sources to be a Class 0 candidate (MMS3) and a double Class I (MMS2).
For the Class II population we find a best age of ~ 2 Myr and compatibility
with recent Initial Mass Functions (IMFs) by comparing the observed Class II
luminosity function (LF), which is complete to 0.08 L_sun, to various model LFs
with different star formation scenarios and input IMFs.Comment: 18 pages, 16 figures, 3 online tables, accepted by A&
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