486 research outputs found
Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars
Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form "hypermassive" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.Comment: 20 pages, 19 figures, 3 tables, AASTeX, accepted by Ap
Single and Multi-Layered Thin Film Oxides for Potential Fuel Cell Applications
Recently there has been a great level of interest in the effect of interfaces in
oxide ionic conductors with a view to eventual application in solid oxide fuel
cells (SOFCs). Enhancements in electrical conductivity of one half to eight orders
of magnitude have been reported in simplified thin film and multi-layered
heterostructure systems. Often this is reported to be enhanced oxygen ion conduction
with little supporting evidence. The aim of this work is to investigate
these reports and develop an understanding of the underlying mechanisms.
Several series of samples were investigated to achieve this goal. The first, designed
to emulate an anomalous result from literature with alternating samarium
doped ceria (SDC) and undoped ceria layers, featured an increasing total number
of layers of equal individual thickness, and thus a constant interfacial density. This
system featured no enhancement in conductivity and exhibited a level of tracer
diffusion comparable with that in bulk SDC. Electron energy loss spectroscopy
(EELS) studies, however, revealed a significant level of Ce III in the undoped layers.
The second and third series used similar materials but tested the hypothesis
proposed in many works, that conductivity enhancement was related to tensile
strain in the conducting material at the heterointerfaces. The second, manipulating
the strain at the interface by varying the dopant (Nd, Sm, Y) in films with
alternating doped and undoped ceria layers and a range of interfacial densities.
This series exhibited minimal change in conductivity with strain or interfacial
density.
The third series replaced the doped ceria with yttria-stabilised zirconia (YSZ)
in order to achieve a higher level of tensile strain. This again featured minimal
change in conductivity. Tracer diffusion and secondary ion mass spectrometry
(SIMS) studies suggested that the undoped ceria layers featured vacancy-rich
regions, close to the interfaces, possibly with compensating Ce III.
The final series of multilayers comprised alternating praseodymium nickel copper
gallate (Pr1.91 Ni0.71 Cu0.24 Ga0.05 O4) and SDC layers which exhibited a high
level of conductivity and evidence of reduced levels of p-type conduction with
decreasing SDC layer thickness, suggesting enhanced ionic conductivity. Oxygen
tracer studies revealed, however, that the dominant charge carrier was not oxygen.
Finally a study of the effect of dislocations in ionic conductors was performed on
deformed single crystal YSZ. Impedance measurements revealed a small enhancement
in conductivity in the orientation parallel to the dislocation cores however
diffusion measurements showed a change that could be negated by the consideration
of the inherent errors
Implementing an apparent-horizon finder in three dimensions
Locating apparent horizons is not only important for a complete understanding
of numerically generated spacetimes, but it may also be a crucial component of
the technique for evolving black-hole spacetimes accurately. A scheme proposed
by Libson et al., based on expanding the location of the apparent horizon in
terms of symmetric trace-free tensors, seems very promising for use with
three-dimensional numerical data sets. In this paper, we generalize this scheme
and perform a number of code tests to fully calibrate its behavior in
black-hole spacetimes similar to those we expect to encounter in solving the
binary black-hole coalescence problem. An important aspect of the
generalization is that we can compute the symmetric trace-free tensor expansion
to any order. This enables us to determine how far we must carry the expansion
to achieve results of a desired accuracy. To accomplish this generalization, we
describe a new and very convenient set of recurrence relations which apply to
symmetric trace-free tensors.Comment: 14 pages (RevTeX 3.0 with 3 figures
Stability and collapse of rapidly rotating, supramassive neutron stars: 3D simulations in general relativity
We perform 3D numerical simulations in full general relativity to study the
stability of rapidly rotating, supramassive neutron stars at the mass-shedding
limit to dynamical collapse. We adopt an adiabatic equation of state with
and focus on uniformly rotating stars. We find that the onset of
dynamical instability along mass-shedding sequences nearly coincides with the
onset of secular instability. Unstable stars collapse to rotating black holes
within about one rotation period. We also study the collapse of stable stars
which have been destabilized by pressure depletion (e.g. via a phase
transition) or mass accretion. In no case do we find evidence for the formation
of massive disks or any ejecta around the newly formed Kerr black holes, even
though the progenitors are rapidly rotating.Comment: 16 pages, to appear in Phys. Rev.
Efficacy and Safety of Vancomycin Loading Doses in Critically Ill Patients with Methicillin-Resistant \u3ci\u3eStaphylococcus aureus\u3c/i\u3e Infection
Background: While vancomycin loading doses may facilitate earlier pharmacokinetic–pharmacodynamic target attainment, the impact of loading doses on clinical outcomes remains understudied. Critically ill patients are at highest risk of morbidity and mortality from methicillin resistant Staphylococcus aureus (MRSA) infection and hypothesized to most likely benefit from a loading dose. We sought to determine the association between receipt of a vancomycin loading dose and clinical outcomes in a cohort of critically ill adults.
Methods: Four hundred and forty-nine critically ill patients with MRSA cultures isolated from blood or respiratory specimens were eligible for the study. Cohorts were established by receipt of a loading dose (⩾20 mg/kg actual body weight) or not. The primary outcome was clinical failure, a composite outcome of death within 30 days of first MRSA culture, blood cultures positive ⩾7 days, white blood cell count up to 5 days from vancomycin initiation, temperature up to 5 days from vancomycin initiation, or substitution (or addition) of another MRSA agent.
Results: There was no difference in the percentage of patients experiencing clinical failure between the loading dose and no loading dose groups (74.8% versus 72.8%; p = 0.698). Secondary outcomes were also similar between groups, including mortality and acute kidney injury, as was subgroup analysis based on site of infection. Exploratory analyses, including assessment of loading dose based on quartiles and a multivariable logistic regression model showed no differences.
Conclusion: Use of vancomycin loading doses was not associated with improved clinical outcomes in critically ill patients with MRSA infection
Far Ultraviolet Absolute Flux of alpha Virginis
We present the far ultraviolet spectrum of alpha Virginis taken with EURD
spectrograph on-board MINISAT-01. The spectral range covered is from ~900 to
1080 A with 5 A spectral resolution. We have fitted Kurucz models to IUE
spectra of alpha Vir and compared the extension of the model to our wavelengths
with EURD data. This comparison shows that EURD fluxes are consistent with the
prediction of the model within 20-30%, depending on the reddening assumed. EURD
fluxes are consistent with Voyager observations but are ~60% higher than most
previous rocket observations of alpha Vir.Comment: 13 pages, 4 figures. Submitted to The Astrophysical Journa
Computing the Complete Gravitational Wavetrain from Relativistic Binary Inspiral
We present a new method for generating the nonlinear gravitational wavetrain
from the late inspiral (pre-coalescence) phase of a binary neutron star system
by means of a numerical evolution calculation in full general relativity. In a
prototype calculation, we produce 214 wave cycles from corotating polytropes,
representing the final part of the inspiral phase prior to reaching the ISCO.
Our method is based on the inequality that the orbital decay timescale due to
gravitational radiation is much longer than an orbital period and the
approximation that gravitational radiation has little effect on the structure
of the stars. We employ quasi-equilibrium sequences of binaries in circular
orbit for the matter source in our field evolution code. We compute the
gravity-wave energy flux, and, from this, the inspiral rate, at a discrete set
of binary separations. From these data, we construct the gravitational waveform
as a continuous wavetrain. Finally, we discuss the limitations of our current
calculation, planned improvements, and potential applications of our method to
other inspiral scenarios.Comment: 4 pages, 4 figure
Cauchy-perturbative matching and outer boundary conditions: computational studies
We present results from a new technique which allows extraction of
gravitational radiation information from a generic three-dimensional numerical
relativity code and provides stable outer boundary conditions. In our approach
we match the solution of a Cauchy evolution of the nonlinear Einstein field
equations to a set of one-dimensional linear equations obtained through
perturbation techniques over a curved background. We discuss the validity of
this approach in the case of linear and mildly nonlinear gravitational waves
and show how a numerical module developed for this purpose is able to provide
an accurate and numerically convergent description of the gravitational wave
propagation and a stable numerical evolution.Comment: 20 pages, RevTe
Equilibrium and stability of supermassive stars in binary systems
We investigate the equilibrium and stability of supermassive stars of mass M
\agt 10^5M_{\odot} in binary systems. We find that corotating binaries are
secularly unstable for close, circular orbits with r \alt
4R(M/10^6M_{\odot})^{1/6} where is the orbital separation and the
stellar radius. We also show that corotation cannot be achieved for distant
orbits with r \agt 12 R (M/10^6M_{\odot})^{-11/24}, since the timescale for
viscous angular momentum transfer associated with tidal torques is longer than
the evolution timescale due to emission of thermal radiation. These facts
suggest that the allowed mass range and orbital separation for corotating
supermassive binary stars is severely restricted. In particular, for
supermassive binary stars of large mass M \agt 6\times 10^6M_{\odot},
corotation cannot be achieved, as viscosity is not adequate to mediate the
transfer between orbital and spin angular momentum. One possible outcome for
binary supermassive stars is the onset of quasi-radial, relativistic
instability which drives each star to collapse prior to merger: We discuss
alternative outcomes of collapse and possible spin states of the resulting
black holes. We estimate the frequency and amplitude of gravitational waves
emitted during several inspiral and collapse scenarios.Comment: 20 pages, to be published in PR
SARS-CoV-2 anti-spike IgG antibody responses after second dose of ChAdOx1 or BNT162b2 and correlates of protection in the UK general population
Antibody responses are an important part of immunity after Coronavirus Disease 2019 (COVID-19) vaccination. However, antibody trajectories and the associated duration of protection after a second vaccine dose remain unclear. In this study, we investigated anti-spike IgG antibody responses and correlates of protection after second doses of ChAdOx1 or BNT162b2 vaccines for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the United Kingdom general population. In 222,493 individuals, we found significant boosting of anti-spike IgG by the second doses of both vaccines in all ages and using different dosing intervals, including the 3-week interval for BNT162b2. After second vaccination, BNT162b2 generated higher peak levels than ChAdOX1. Older individuals and males had lower peak levels with BNT162b2 but not ChAdOx1, whereas declines were similar across ages and sexes with ChAdOX1 or BNT162b2. Prior infection significantly increased antibody peak level and half-life with both vaccines. Anti-spike IgG levels were associated with protection from infection after vaccination and, to an even greater degree, after prior infection. At least 67% protection against infection was estimated to last for 2–3 months after two ChAdOx1 doses, for 5–8 months after two BNT162b2 doses in those without prior infection and for 1–2 years for those unvaccinated after natural infection. A third booster dose might be needed, prioritized to ChAdOx1 recipients and those more clinically vulnerable
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