2,193 research outputs found
Multi-chord fiber-coupled interferometer with a long coherence length laser
This paper describes a 561 nm laser heterodyne interferometer that provides
time-resolved measurements of line-integrated plasma electron density within
the range of 10^15-10^18 cm^(-2). Such plasmas are produced by railguns on the
Plasma Liner Experiment (PLX), which aims to produce \mu s-, cm-, and
Mbar-scale plasmas through the merging of thirty plasma jets in a spherically
convergent geometry. A long coherence length, 320 mW laser allows for a strong,
sub-fringe phase-shift signal without the need for closely-matched probe and
reference path lengths. Thus only one reference path is required for all eight
probe paths, and an individual probe chord can be altered without altering the
reference or other probe path lengths. Fiber-optic decoupling of the probe
chord optics on the vacuum chamber from the rest of the system allows the probe
paths to be easily altered to focus on different spatial regions of the plasma.
We demonstrate that sub-fringe resolution capability allows the interferometer
to operate down to line-integrated densities of order 10^15 cm^(-2).Comment: submitted to Rev. Sci. Instrum. (2011
Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Low-Mass Stellar Initial Mass Function at High Redshift
The stellar initial mass function at high redshift is an important defining
property of the first stellar systems to form and may also play a role in
various dark matter problems. We here determine the faint stellar luminosity
function in an apparently dark-matter-dominated external galaxy in which the
stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a
system with a particularly simple stellar population - all of the stars being
old and metal-poor - similar to that of a classical halo globular cluster. A
direct comparison of the faint luminosity functions of the UMi Sph and of
similar metallicity, old globular clusters is equivalent to a comparison of the
initial mass functions and is presented here, based on deep HST WFPC2 and STIS
imaging data. We find that these luminosity functions are indistinguishable,
down to a luminosity corresponding to 0.3 solar masses. Our results show that
the low-mass stellar IMF for stars that formed at very high redshift is
apparently invariant across environments as diverse as those of an extremely
low-surface-brightness, dark-matter-dominated dwarf galaxy and a
dark-matter-free, high-density globular cluster within the Milky Way.Comment: Accepted by New Astronomy. 64 pages, including 9 embedded postscript
tables and 20 embedded postscript figures, plus 14 separate jpeg figures.
Postscript versions of the jpeg figures and a complete version of the paper
with all figures embedded can be found at http://tarkus.pha.jhu.edu/~mlh
Core radius evolution of star clusters
We use N-body simulations of star clusters to investigate the possible
dynamical origins of the observed spread in core radius among intermediate-age
and old star clusters in the Large Magellanic Cloud (LMC). Two effects are
considered, a time-varying external tidal field and variations in primordial
hard binary fraction. Simulations of clusters orbiting a point-mass galaxy show
similar core radius evolution for clusters on both circular and elliptical
orbits and we therefore conclude that the tidal field of the LMC has not yet
significantly influenced the evolution of the intermediate-age clusters. The
presence of large numbers of hard primordial binaries in a cluster leads to
core radius expansion; however, the magnitude of the effect is insufficient to
explain the observations. Further, the range of binary fractions required to
produce significant core radius growth is inconsistent with the observational
evidence that all the LMC clusters have similar stellar luminosity functions.Comment: Accepted for publication in MNRA
Evaluation of work-based screening for early signs of alcohol-related liver disease in hazardous and harmful drinkers: the PrevAIL study
Background
The direct cost of excessive alcohol consumption to health services is substantial but dwarfed by the cost borne by the workplace as a result of lost productivity. The workplace is also a promising setting for health interventions. The Preventing Alcohol Harm in Liverpool and Knowsley (PrevAIL) project aimed to evaluate a mechanism for detecting the prevalence of alcohol related liver disease using fibrosis biomarkers. Secondary aims were to identify the additive effect of obesity as a risk factor for early liver disease; to assess other impacts of alcohol on work, using a cross-sectional survey.
Methods
Participants (aged 36-55y) from 13 workplaces participated (March 2011–April 2012). BMI, waist circumference, blood pressure and self-reported alcohol consumption in the previous week was recorded. Those consuming more than the accepted UK threshold (men: >21 units; female: >14 units alcohol) provided a 20 ml venous blood sample for a biomarker test (Southampton Traffic Light Test) and completed an alcohol questionnaire (incorporating the Severity of Alcohol Dependence Questionnaire).
Results
The screening mechanism enrolled 363 individuals (52 % women), 39 % of whom drank above the threshold and participated in the liver screen (n = 141, complete data = 124 persons). Workplaces with successful participation were those where employers actively promoted, encouraged and facilitated attendance. Biomarkers detected that 30 % had liver disease (25 %, intermediate; 5 % probable). Liver disease was associated with the frequency of visits to the family physician (P = 0.036) and obesity (P = 0.052).
Conclusions
The workplace is an important setting for addressing alcohol harm, but there are barriers to voluntary screening that need to be addressed. Early detection and support of cases in the community could avert deaths and save health and social costs. Alcohol and obesity should be addressed simultaneously, because of their known multiplicative effect on liver disease risk, and because employers preferred a general health intervention to one that focused solely on alcohol consumption
High Spectral Resolution Observations of the Massive Stars in the Galactic Center
We present high-resolution near-infrared spectra, obtained with the NIRSPEC
spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive
stars within the central 25 arcseconds of the Galactic center. We have
identified a total of twenty-one emission-line stars, seven of which are new
radial velocity detections with five of those being classified as He I
emission-line stars for the first time. These stars fall into two categories
based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He
I doublet absorption lines, and 2) those with broad 2.058 micron He I emission
lines. These data have the highest spectral resolution ever obtained for these
sources and, as a result, both components of the absorption doublet are
separately resolved for the first time. We use these spectral features to
measure radial velocities. The majority of the measured radial velocities have
relative errors of 20 kms, smaller than those previously obtained with
proper-motion or radial velocity measurements for similar stellar samples in
the Galactic center. The radial velocities estimated from the He I absorption
doublet are more robust than those previously estimated from the 2.058 micron
emission line, since they do not suffer from confusion due to emission from the
surrounding ISM. Using this velocity information, we agree that the stars are
orbiting in a somewhat coherent manner but are not as defined into a disk or
disks as previously thought. Finally, multi-epoch radial velocity measurements
for IRS 16NE show a change in its velocity presumably due to an unseen stellar
companion.Comment: ApJ accepted, 42 pages, 16 figure
Multi-chord fiber-coupled interferometry of supersonic plasma jets and comparisons with synthetic data
A multi-chord fiber-coupled interferometer [Merritt et al., Rev. Sci.
Instrum. 83, 033506 (2012)] is being used to make time-resolved density
measurements of supersonic argon plasma jets on the Plasma Liner Experiment
[Hsu et al., Bull. Amer. Phys. Soc. 56, 307 (2011)]. The long coherence length
of the laser (>10 m) allows signal and reference path lengths to be mismatched
by many meters without signal degradation, making for a greatly simplified
optical layout. Measured interferometry phase shifts are consistent with a
partially ionized plasma in which an initially positive phase shift becomes
negative when the ionization fraction drops below a certain threshold. In this
case, both free electrons and bound electrons in ions and neutral atoms
contribute to the index of refraction. This paper illustrates how the
interferometry data, aided by numerical modeling, are used to derive total jet
density, jet propagation velocity (~15-50 km/s), jet length (~20-100 cm), and
3D expansion.Comment: 6 pages, 4 figures, invited paper at the 19th High Temperature Plasma
Diagnostics Conference, Monterey, CA, May 6--10, 201
Holomorphic Hartree-Fock Theory: The Nature of Two-Electron Problems.
We explore the existence and behavior of holomorphic restricted Hartree-Fock (h-RHF) solutions for two-electron problems. Through algebraic geometry, the exact number of solutions with n basis functions is rigorously identified as 1/2(3n - 1), proving that states must exist for all molecular geometries. A detailed study on the h-RHF states of HZ (STO-3G) then demonstrates both the conservation of holomorphic solutions as geometry or atomic charges are varied and the emergence of complex h-RHF solutions at coalescence points. Using catastrophe theory, the nature of these coalescence points is described, highlighting the influence of molecular symmetry. The h-RHF states of HHeH2+ and HHeH (STO-3G) are then compared, illustrating the isomorphism between systems with two electrons and two electron holes. Finally, we explore the h-RHF states of ethene (STO-3G) by considering the π electrons as a two-electron problem and employ NOCI to identify a crossing of the lowest energy singlet and triplet states at the perpendicular geometry
The alcohol harm paradox: using a national survey to explore how alcohol may disproportionately impact health in deprived individuals
Background Internationally, studies show that similar levels of alcohol consumption in deprived communities (vs. more affluent) result in higher levels of alcohol-related ill health. Hypotheses to explain this alcohol harm paradox include deprived drinkers: suffering greater combined health challenges (e.g. smoking, obesity) which exacerbate effects of alcohol harms; exhibiting more harmful consumption patterns (e.g. bingeing); having a history of more harmful consumption; and disproportionately under-reporting consumption. We use a bespoke national survey to assess each of these hypotheses. Methods A national telephone survey designed to test this alcohol harm paradox was undertaken (May 2013 to April 2014) with English adults (n = 6015). Deprivation was assigned by area of residence. Questions examined factors including: current and historic drinking patterns; combined health challenges (smoking, diet, exercise and body mass); and under-reported consumption (enhanced questioning on atypical/special occasion drinking). For each factor, analyses examined differences between deprived and more affluent individuals controlled for total alcohol consumption. Results Independent of total consumption, deprived drinkers were more likely to smoke, be overweight and report poor diet and exercise. Consequently, deprived increased risk drinkers (male >168–400 g, female >112–280 g alcohol/week) were >10 times more likely than non-deprived counterparts to drink in a behavioural syndrome combining smoking, excess weight and poor diet/exercise. Differences by deprivation were significant but less marked in higher risk drinkers (male >400 g, female >280 g alcohol/week). Current binge drinking was associated with deprivation independently of total consumption and a history of bingeing was also associated with deprivation in lower and increased risk drinkers. Conclusions Deprived increased/higher drinkers are more likely than affluent counterparts to consume alcohol as part of a suite of health challenging behaviours including smoking, excess weight and poor diet/exercise. Together these can have multiplicative effects on risks of wholly (e.g. alcoholic liver disease) and partly (e.g. cancers) alcohol-related conditions. More binge drinking in deprived individuals will also increase risks of injury and heart disease despite total alcohol consumption not differing from affluent counterparts. Public health messages on how smoking, poor diet/exercise and bingeing escalate health risks associated with alcohol are needed, especially in deprived communities, as their absence will contribute to health inequalities
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Feasibility of In Situ Redox Manipulation of Subsurface Sediments for RDX Remediation at Pantex
This laboratory study was conducted to assess RDX (hexahydro-1,3,5-trinitro-1,3,5 triazine) abiotic degradation by chemically reduced sediments and other geochemical aspects of the application of this technology to remediation of RDX contamination in groundwater at the U.S. DOE Pantex facility..
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