373 research outputs found
A Detailed Comparison of Multi-Dimensional Boltzmann Neutrino Transport Methods in Core-Collapse Supernovae
The mechanism driving core-collapse supernovae is sensitive to the interplay
between matter and neutrino radiation. However, neutrino radiation transport is
very difficult to simulate, and several radiation transport methods of varying
levels of approximation are available. We carefully compare for the first time
in multiple spatial dimensions the discrete ordinates (DO) code of Nagakura,
Yamada, and Sumiyoshi and the Monte Carlo (MC) code Sedonu, under the
assumptions of a static fluid background, flat spacetime, elastic scattering,
and full special relativity. We find remarkably good agreement in all spectral,
angular, and fluid interaction quantities, lending confidence to both methods.
The DO method excels in determining the heating and cooling rates in the
optically thick region. The MC method predicts sharper angular features due to
the effectively infinite angular resolution, but struggles to drive down noise
in quantities where subtractive cancellation is prevalent, such as the net gain
in the protoneutron star and off-diagonal components of the Eddington tensor.
We also find that errors in the angular moments of the distribution functions
induced by neglecting velocity dependence are sub-dominant to those from
limited momentum-space resolution. We briefly compare directly computed second
angular moments to those predicted by popular algebraic two-moment closures,
and find that the errors from the approximate closures are comparable to the
difference between the DO and MC methods. Included in this work is an improved
Sedonu code, which now implements a fully special relativistic,
time-independent version of the grid-agnostic Monte Carlo random walk
approximation.Comment: Accepted to ApJS. 24 pages, 19 figures. Key simulation results and
codes are available at https://stellarcollapse.org/MCvsD
Nonequilibrium electron spin polarization in a double quantum dot. Lande mechanism
In moderately strong magnetic fields, the difference in Lande g-factors in
each of the dots of a coupled double quantum dot device may induce oscillations
between singlet and triplet states of the entangled electron pair and lead to a
nonequilibrium electron spin polarization. We will show that this polarization
may partially survive the rapid inhomogeneous decoherence due to random nuclear
magnetic fields.Comment: New version contains figures. New title better reflects the content
of the pape
The potential for detecting gamma-ray burst afterglows from population III stars with the next generation of infrared telescopes
We investigate the detectability of a proposed population of gamma-ray bursts (GRBs) from the collapse of Population III (Pop III) stars. The James Webb Space Telescope (JWST) and Space Infrared Telescope for Cosmology and Astrophysics (SPICA) will be able to observe the late time infrared afterglows. We have developed a new method to calculate their detectability, which takes into account the fundamental initial mass function and formation rates of Pop III stars, from which we find the temporal variability of the afterglows and ultimately the length of time JWST and SPICA can detect them. In the range of plausible Pop III GRB parameters, the afterglows are always detectable by these instruments during the isotropic emission, for a minimum of 55 days and a maximum of 3.7 yr. The average number of detectable afterglows will be 2.96× 10–5 per SPICA field of view (FOV) and 2.78× 10–6 per JWST FOV. These are lower limits, using a pessimistic estimate of Pop III star formation. An optimal observing strategy with SPICA could identify a candidate orphan afterglow in ~1.3 yr, with a 90% probability of confirmation with further detailed observations. A beamed GRB will align with the FOV of the planned GRB detector Energetic X-ray Imaging Survey Telescope once every 9 yr. Pop III GRBs will be more easily detected by their isotropic emissions (i.e., orphan afterglows) rather than by their prompt emissions
General Relativistic Hydrodynamic Simulations and Linear Analysis of the Standing Accretion Shock Instability around a Black Hole
We study the stability of standing shock waves in advection-dominated
accretion flows into a Schwarzschild black hole by 2D general relativistic
hydrodynamic simulations as well as linear analysis in the equatorial plane. We
demonstrate that the accretion shock is stable against axisymmetric
perturbations but becomes unstable to non-axisymmetric perturbations. The
results of dynamical simulations show good agreement with linear analysis on
the stability, oscillation and growing time scales. The comparison of different
wave-travel times with the growth time scales of the instability suggests that
the instability is likely to be of the Papaloizou-Pringle type, induced by the
repeated propagations of acoustic waves. However, the wavelengths of
perturbations are too long to clearly define the reflection point. By analyzing
the non-linear phase in the dynamical simulations, it is shown that quadratic
mode couplings precede the non-linear saturation. It is also found that not
only short-term random fluctuations by turbulent motions but also quasi
periodic oscillations take place on longer time scales in the non-linear phase.
We give some possible implications of the instability for quasi periodic
oscillations (QPOs) and the central engine for gamma ray bursts (GRBs).Comment: 34 pages, 11 figures, accepted in Ap
String amplitudes in arbitrary dimensions
We calculate gravitational dressed tachyon correlators in non critcal
dimensions. The 2D gravity part of our theory is constrained to constant
curvature. Then scaling dimensions of gravitational dressed vertex operators
are equal to their bare conformal dimensions. Considering the model as d+2
dimensional critical string we calculate poles of generalized Shapiro-Virasoro
amplitudes.Comment: 14 page
Chemistry of heavy elements in the Dark Ages
Primordial molecules were formed during the Dark Ages, i.e. the time between
recombination and reionization in the early Universe. The purpose of this
article is to analyze the formation of primordial molecules based on heavy
elements during the Dark Ages, with elemental abundances taken from different
nucleosynthesis models. We present calculations of the full non-linear equation
set governing the primordial chemistry. We consider the evolution of 45
chemical species and use an implicit multistep method of variable order of
precision with an adaptive stepsize control. We find that the most abundant
Dark Ages molecules based on heavy elements are CH and OH. Non-standard
nucleosynthesis can lead to higher heavy element abundances while still
satisfying the observed primordial light abundances. In that case, we show that
the abundances of molecular species based on C, N, O and F can be enhanced by
two orders of magnitude compared to the standard case, leading to a CH relative
abundance higher than that of HD+ or H2D+.Comment: 14 pages, accepted by Astronomy and Astrophysic
Neutron lifetime measurement with pulsed cold neutrons
The neutron lifetime has been measured by comparing the decay rate with the
reaction rate of He nuclei of a pulsed neutron beam from the spallation
neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The
decay rate and the reaction rate were determined by simultaneously detecting
electrons from the neutron decay and protons from the He(n,p)H reaction
using a gas chamber of which working gas contains diluted He. The measured
neutron lifetime was 898\,\pm\,10\,_{\rm stat}\,^{+15}_{-18}\,_{\rm sys}\,s.Comment: 28 pages, 20 figures, will be submitted to PTE
Treating asthma with omega-3 fatty acids: where is the evidence? A systematic review
BACKGROUND: Considerable interest exists in the potential therapeutic value of dietary supplementation with the omega-3 fatty acids. Given the interplay between pro-inflammatory omega-6 fatty acids, and the less pro-inflammatory omega-3 fatty acids, it has been thought that the latter could play a key role in treating or preventing asthma. The purpose was to systematically review the scientific-medical literature in order to identify, appraise, and synthesize the evidence for possible treatment effects of omega-3 fatty acids in asthma. METHODS: Medline, Premedline, Embase, Cochrane Central Register of Controlled Trials, CAB Health, and, Dissertation Abstracts were searched to April 2003. We included randomized controlled trials (RCT's) of subjects of any age that used any foods or extracts containing omega-3 fatty acids as treatment or prevention for asthma. Data included all asthma related outcomes, potential covariates, characteristics of the study, design, population, intervention/exposure, comparators, and co interventions. RESULTS: Ten RCT's were found pertinent to the present report. CONCLUSION: Given the largely inconsistent picture within and across respiratory outcomes, it is impossible to determine whether or not omega-3 fatty acids are an efficacious adjuvant or monotherapy for children or adults. Based on this systematic review we recommend a large randomized controlled study of the effects of high-dose encapsulated omega-3 fatty acids on ventilatory and inflammatory measures of asthma controlling diet and other asthma risk factors. This review was limited because Meta-analysis was considered inappropriate due to missing data; poorly or heterogeneously defined populations, interventions, intervention-comparator combinations, and outcomes. In addition, small sample sizes made it impossible to meaningfully assess the impact on clinical outcomes of co-variables. Last, few significant effects were found
The effect of Aloe ferox Mill. in the treatment of loperamide-induced constipation in Wistar rats
<p>Abstract</p> <p>Background</p> <p>Constipation is the most common gastrointestinal complaint all over the world and it is a risk factor of colorectal cancer. In this study, the efficacy of aqueous leaf extract of <it>Aloe </it><it>ferox </it>Mill. was studied against loperamide-induced constipation in Wistar rats.</p> <p>Methods</p> <p>Constipation was induced by oral administration of loperamide (3 mg/kg body weight) while the control rats received normal saline. The constipated rats were treated with 50, 100 and 200 mg/kg body weight/day of the extract for 7 days during which the feeding characteristics, body weight, fecal properties and gastrointestinal transit ratio were monitored.</p> <p>Results</p> <p>The extract improved intestinal motility, increased fecal volume and normalized body weight in the constipated rats, which are indications of laxative property of the herb with the 200 mg/kg body weight of the extract showing the best efficacy.</p> <p>Conclusion</p> <p>The effect of the extract compares favourably well with senokot, a standard laxative drug. These findings have therefore, lent scientific credence to the folkloric use of the herb as a laxative agent by the people of the Eastern Cape of South Africa.</p
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