47,539 research outputs found
The Virginia Sickle Cell Anemia Awareness Program: Education, Screening, and Counseling
In 1968, a program of screening for sickle trait carriers was begun as part of the work of the Hematology Division, Department of Medicine, at the Medical College of Virginia. It was felt that sickle cell anemia was more of a public health problem than was generally recognized, and in addition to instituting screening and education programs, data were collected to document the relative neglect of the problem
Size effects and dislocation patterning in two-dimensional bending
We perform atomistic Monte Carlo simulations of bending a Lennard-Jones
single crystal in two dimensions. Dislocations nucleate only at the free
surface as there are no sources in the interior of the sample. When
dislocations reach sufficient density, they spontaneously coalesce to nucleate
grain boundaries, and the resulting microstructure depends strongly on the
initial crystal orientation of the sample. In initial yield, we find a reverse
size effect, in which larger samples show a higher scaled bending moment than
smaller samples for a given strain and strain rate. This effect is associated
with source-limited plasticity and high strain rate relative to dislocation
mobility, and the size effect in initial yield disappears when we scale the
data to account for strain rate effects. Once dislocations coalesce to form
grain boundaries, the size effect reverses and we find that smaller crystals
support a higher scaled bending moment than larger crystals. This finding is in
qualitative agreement with experimental results. Finally, we observe an
instability at the compressed crystal surface that suggests a novel mechanism
for the formation of a hillock structure. The hillock is formed when a high
angle grain boundary, after absorbing additional dislocations, becomes unstable
and folds to form a new crystal grain that protrudes from the free surface.Comment: 15 pages, 8 figure
Configurations of Extremal Even Unimodular Lattices
We extend the results of Ozeki on the configurations of extremal even
unimodular lattices. Specifically, we show that if L is such a lattice of rank
56, 72, or 96, then L is generated by its minimal-norm vectors.Comment: 8 pages. To appear, International Journal of Number Theor
The Lanczos algorithm with selective orthogonalization
A new stable and efficient implementation of the Lanczos algorithm is presented. The algorithm is a powerful method for finding a few eigenvalues and eigenvectors at one or both ends of the spectrum of a symmetric matrix A. The algorithm is particularly effective if A is large and sparse in that the only way in which A enters the calculation is through a subroutine which computes Av for any vector v. Thus the user is free to take advantage of any sparsity structure in A and A need not even be represented as a matrix et al
Ultraviolet/X-ray variability and the extended X-ray emission of the radio-loud broad absorption line quasar PG 1004+130
We present the results of recent Chandra, XMM-Newton, and Hubble Space
Telescope observations of the radio-loud (RL), broad absorption line (BAL)
quasar PG 1004+130. We compare our new observations to archival X-ray and UV
data, creating the most comprehensive, high signal-to-noise, multi-epoch,
spectral monitoring campaign of a RL BAL quasar to date. We probe for
variability of the X-ray absorption, the UV BAL, and the X-ray jet, on
month-year timescales. The X-ray absorber has a low column density of
cm when it is assumed to be fully
covering the X-ray emitting region, and its properties do not vary
significantly between the 4 observations. This suggests the observed absorption
is not related to the typical "shielding gas" commonly invoked in BAL quasar
models, but is likely due to material further from the central black hole. In
contrast, the CIV BAL shows strong variability. The equivalent width (EW) in
2014 is EW=11.240.56 \AA, showing a fractional increase of =1.160.11 from the 2003 observation, 3183 days earlier
in the rest-frame. This places PG 1004+130 among the most highly variable BAL
quasars. By combining Chandra observations we create an exposure 2.5 times
deeper than studied previously, with which to investigate the nature of the
X-ray jet and extended diffuse X-ray emission. An X-ray knot, likely with a
synchrotron origin, is detected in the radio jet ~8 arcsec (30 kpc) from the
central X-ray source with a spatial extent of ~4 arcsec (15 kpc). No similar
X-ray counterpart to the counterjet is detected. Asymmetric, non-thermal
diffuse X-ray emission, likely due to inverse Compton scattering of Cosmic
Microwave Background photons, is also detected.Comment: 15 pages, 7 figures, 3 tables. Accepted for publication in Ap
Dynamical Origin of Extrasolar Planet Eccentricity Distribution
We explore the possibility that the observed eccentricity distribution of
extrasolar planets arose through planet-planet interactions, after the initial
stage of planet formation was complete. Our results are based on ~3250
numerical integrations of ensembles of randomly constructed planetary systems,
each lasting 100 Myr. We find that for a remarkably wide range of initial
conditions the eccentricity distributions of dynamically active planetary
systems relax towards a common final equilibrium distribution, well described
by the fitting formula dn ~ e exp[-1/2 (e/0.3)^2] de. This distribution agrees
well with the observed eccentricity distribution for e > 0.2, but predicts too
few planets at lower eccentricities, even when we exclude planets subject to
tidal circularization. These findings suggest that a period of large-scale
dynamical instability has occurred in a significant fraction of newly formed
planetary systems, lasting 1--2 orders of magnitude longer than the ~1 Myr
interval in which gas-giant planets are assembled. This mechanism predicts no
(or weak) correlations between semimajor axis, eccentricity, inclination, and
mass in dynamically relaxed planetary systems. An additional observational
consequence of dynamical relaxation is a significant population of planets
(>10%) that are highly inclined (>25deg) with respect to the initial symmetry
plane of the protoplanetary disk; this population may be detectable in
transiting planets through the Rossiter-McLaughlin effect.Comment: Accepted to ApJ, conclusions updated to reflect the current
observational constraint
Correlation length scalings in fusion edge plasma turbulence computations
The effect of changes in plasma parameters, that are characteristic near or
at an L-H transition in fusion edge plasmas, on fluctuation correlation lengths
are analysed by means of drift-Alfven turbulence computations. Scalings by
density gradient length, collisionality, plasma beta, and by an imposed shear
flow are considered. It is found that strongly sheared flows lead to the
appearence of long-range correlations in electrostatic potential fluctuations
parallel and perpendicular to the magnetic field.Comment: Submitted to "Plasma Physics and Controlled Fusion
The influence of defects of the fatigue resistance of butt and girth welds in A106B steel
This three-phase study was directed at developing a fitness for service defect acceptance criteria for welds with defect indications. The study focussed on A106 Gr. B steel pipe. The first phase involved a literature search and critical review to develop the preliminary acceptance criteria to the extent permitted by the data. The second phase developed data for flat plate, wall segment, and vessel specimens containing artificial or natural planar or volumetric defects. The final phase developed acceptance criteria from the test data
A microrod-resonator Brillouin laser with 240 Hz absolute linewidth
We demonstrate an ultralow-noise microrod-resonator based laser that
oscillates on the gain supplied by the stimulated Brillouin scattering optical
nonlinearity. Microresonator Brillouin lasers are known to offer an outstanding
frequency noise floor, which is limited by fundamental thermal fluctuations.
Here, we show experimental evidence that thermal effects also dominate the
close-to-carrier frequency fluctuations. The 6-mm diameter microrod resonator
used in our experiments has a large optical mode area of ~100 {\mu}m, and
hence its 10 ms thermal time constant filters the close-to-carrier optical
frequency noise. The result is an absolute laser linewidth of 240 Hz with a
corresponding white-frequency noise floor of 0.1 Hz/Hz. We explain the
steady-state performance of this laser by measurements of its operation state
and of its mode detuning and lineshape. Our results highlight a mechanism for
noise that is common to many microresonator devices due to the inherent
coupling between intracavity power and mode frequency. We demonstrate the
ability to reduce this noise through a feedback loop that stabilizes the
intracavity power.Comment: 11 pages, 5 figure
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