12,432 research outputs found
Dilatation of Lateral Ventricles with Brain Volumes in Infants with 3D Transfontanelle US
Ultrasound (US) can be used to assess brain development in newborns, as MRI
is challenging due to immobilization issues, and may require sedation.
Dilatation of the lateral ventricles in the brain is a risk factor for poorer
neurodevelopment outcomes in infants. Hence, 3D US has the ability to assess
the volume of the lateral ventricles similar to clinically standard MRI, but
manual segmentation is time consuming. The objective of this study is to
develop an approach quantifying the ratio of lateral ventricular dilatation
with respect to total brain volume using 3D US, which can assess the severity
of macrocephaly. Automatic segmentation of the lateral ventricles is achieved
with a multi-atlas deformable registration approach using locally linear
correlation metrics for US-MRI fusion, followed by a refinement step using
deformable mesh models. Total brain volume is estimated using a 3D ellipsoid
modeling approach. Validation was performed on a cohort of 12 infants, ranging
from 2 to 8.5 months old, where 3D US and MRI were used to compare brain
volumes and segmented lateral ventricles. Automatically extracted volumes from
3D US show a high correlation and no statistically significant difference when
compared to ground truth measurements. Differences in volume ratios was 6.0 +/-
4.8% compared to MRI, while lateral ventricular segmentation yielded a mean
Dice coefficient of 70.8 +/- 3.6% and a mean absolute distance (MAD) of 0.88
+/- 0.2mm, demonstrating the clinical benefit of this tool in paediatric
ultrasound
The effect of stellar-mass black holes on the structural evolution of massive star clusters
We present the results of realistic N-body modelling of massive star clusters
in the Magellanic Clouds, aimed at investigating a dynamical origin for the
radius-age trend observed in these systems. We find that stellar-mass black
holes, formed in the supernova explosions of the most massive cluster stars,
can constitute a dynamically important population. If a significant number of
black holes are retained (here we assume complete retention), these objects
rapidly form a dense core where interactions are common, resulting in the
scattering of black holes into the cluster halo, and the ejection of black
holes from the cluster. These two processes heat the stellar component,
resulting in prolonged core expansion of a magnitude matching the observations.
Significant core evolution is also observed in Magellanic Cloud clusters at
early times. We find that this does not result from the action of black holes,
but can be reproduced by the effects of mass-loss due to rapid stellar
evolution in a primordially mass segregated cluster.Comment: Accepted for publication in MNRAS Letters; 2 figures, 1 tabl
The Emergence of the Thick Disk in a CDM Universe II: Colors and Abundance Patterns
The recently emerging conviction that thick disks are prevalent in disk
galaxies, and their seemingly ubiquitous old ages, means that the formation of
the thick disk, perhaps more than any other component, holds the key to
unravelling the evolution of the Milky Way, and indeed all disk galaxies. In
Paper I, we proposed that the thick disk was formed in an epoch of gas rich
mergers, at high redshift. This hypothesis was based on comparing N-body/SPH
simulations to a variety of Galactic and extragalactic observations, including
stellar kinematics, ages and chemical properties.Here examine our thick disk
formation scenario in light of the most recent observations of extragalactic
thick disks. In agreement, our simulted thick disks are old and relatively
metal rich, with V-I colors that do not vary significantly with distance from
the plane. Further, we show that our proposal results in an enhancement of
alpha-elements in thick disk stars as compared with thin disk stars, consistent
with observations of the relevant populations of the Milky Way. We also find
that our scenario naturally leads to the formation of an old metal weak stellar
halo population with high alpha-element abundances.Comment: submitted to Ap
Kinematics of Nearby Subdwarf Stars
We present an analysis of the space motions of 742 subdwarf stars based on
the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper
motions and Tycho2 proper motions were combined with radial velocities and
metallicities from CLLA. The kinematical behavior is discussed in particular in
relation to their metallicities. The majority of these sample stars have metal
abundances of [Fe/H] >-1 and represent the thick disk population. The halo
component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity
and a radially elongated velocity ellipsoid. In the intermediate metallicity
range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with
disklike kinematics. We interpret this population of stars as a metal-weak
thick disk population.Comment: 6 pages, 7 figures, accepted by Astronomy & Astrophysic
The Inner Galaxy resolved at IJK using DENIS data
We present the analysis of three colour optical/near-infrared images, in IJK,
taken for the DENIS project. The region considered covers 17.4 square deg and
lies within |l|<5 deg, |b|<1.5 deg. The adopted methods for deriving photometry
and astrometry in these crowded images, together with an analysis of the
deficiencies nevertheless remaining, are presented. The numbers of objects
extracted in I,J and K are 748000, 851000 and 659000 respectively, to magnitude
limits of 17,15 and 13. 80% completeness levels typically fall at magnitudes
16, 13 and 10 respectively, fainter by about 2 magnitudes than the usual DENIS
limits due to the crowded nature of these fields. A simple model to describe
the disk contribution to the number counts is constructed, and parameters for
the dust layer derived. We find that a formal fit of parameters for the dust
plane, from these data in limited directions, gives a scalelength and
scaleheight of 3.4+-1.0 kpc and 40+-5 pc respectively, and a solar position
14.0+-2.5 pc below the plane. This latter value is likely to be affected by
localised dust asymmetries. We convolve a detailed model of the systematic and
random errors in the photometry with a simple model of the Galactic disk and
dust distribution, to simulate expected colour-magnitude diagrams. These are in
good agreement with the observed diagrams, allowing us to isolate those stars
from the inner disk and bulge. After correcting for local dust-induced
asymmetries, we find evidence for longitude-dependent asymmetries in the
distant J and K sources, consistent with the general predictions of some
Galactic bar models. We consider complementary L-band observations in a second
paper.Comment: 14 pages, 33 figures, LaTeX, MNRAS accepte
The Physical Basis for Long-lived Electronic Coherence in Photosynthetic Light Harvesting Systems
The physical basis for observed long-lived electronic coherence in
photosynthetic light-harvesting systems is identified using an analytically
soluble model. Three physical features are found to be responsible for their
long coherence lifetimes: i) the small energy gap between excitonic states, ii)
the small ratio of the energy gap to the coupling between excitonic states, and
iii) the fact that the molecular characteristics place the system in an
effective low temperature regime, even at ambient conditions. Using this
approach, we obtain decoherence times for a dimer model with FMO parameters of
160 fs at 77 K and 80 fs at 277 K. As such, significant
oscillations are found to persist for 600 fs and 300 fs, respectively, in
accord with the experiment and with previous computations. Similar good
agreement is found for PC645 at room temperature, with oscillations persisting
for 400 fs. The analytic expressions obtained provide direct insight into the
parameter dependence of the decoherence time scales.Comment: 5 figures; J. Phys. Chem. Lett. (2011
Disentangling the Hercules stream
Using high-resolution spectra of nearby F and G dwarf stars, we have
investigated the detailed abundance and age structure of the Hercules stream.
We find that the stars in the stream have a wide range of stellar ages,
metallicities, and element abundances. By comparing to existing samples of
stars in the solar neighbourhood with kinematics typical of the Galactic thin
and thick disks we find that the properties of the Hercules stream distinctly
separate into the abundance and age trends of the two disks. Hence, we find it
unlikely that the Hercules stream is a unique Galactic stellar population, but
rather a mixture of thin and thick disk stars. This points toward a dynamical
origin for the Hercules stream, probably caused by the Galactic bar.Comment: Accepted for publication in ApJ Letter
Contamination control program plan for the ultraviolet spectrometer experiment, revision E
The contamination control program plan delineates the cleanliness requirements to be attained and maintained, and the methods to be utilized, in the fabrication, handling, test, calibration, shipment, pre-installation checkout and installation for the ultraviolet spectrometer experiment prototype, qualification and flight equipment
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