6,170 research outputs found
The United Kingdom National Audit Projects: a narrative review
The Royal College of Anaesthetistsâ National Audit Project (NAP) programme has been running in its current form since 2006. Since NAP3 was commissioned the NAPs have examined rare but important complications of anaesthesia and related subspecialties. The topics covered include major complications of central neuraxial block (NAP3), major complications of airway management in hospitals (NAP4) and accidental awareness during general anaesthesia (NAP5). NAP6 is currently studying severe perioperative anaphylaxis. The NAPs have shed new light on the major complications of anaesthesia, providing both quantitative (frequencies, prevalence, incidence, risk factors) and qualitative (themes, patient stories, human factors) knowledge that has led to new learning, recommendations and changes in practice. This article describes the background, nature and processes of the NAPs.Keywords: Audit, Anaesthesia, Central neuraxial blockade, Airway, Awareness, Complication
Towards a wave--extraction method for numerical relativity: III. Analytical examples for the Beetle--Burko radiation scalar
Beetle and Burko recently introduced a background--independent scalar
curvature invariant for general relativity that carries information only about
the gravitational radiation in generic spacetimes, in cases where such
radiation is incontrovertibly defined. In this paper we adopt a formalism that
only uses spatial data as they are used in numerical relativity and compute the
Beetle--Burko radiation scalar for a number of analytical examples,
specifically linearized Einstein--Rosen cylindrical waves, linearized
quadrupole waves, the Kerr spacetime, Bowen--York initial data, and the Kasner
spacetime. These examples illustrate how the Beetle--Burko radiation scalar can
be used to examine the gravitational wave content of numerically generated
spacetimes, and how it may provide a useful diagnostic for initial data sets.Comment: 23 pages, 4 figures; We changed the convention used, corrected typos,
and expanded the discussio
Quantifying surface water, porewater, and groundwater interactions using tracers: tracer fluxes, water fluxes, and endâmember concentrations
Tracer approaches to estimate both porewater exchange (the cycling of water between surface water and sediments, with zero net water flux) and groundwater inflow (the net flow of terrestrially derived groundwater into surface water) are commonly based on solute mass balances. However, this requires appropriate characterization of tracer endâmember concentrations in exchanging or discharging water. Where either porewater exchange or groundwater inflow to surface water occur in isolation, then the water flux is easily estimated from the net tracer flux if the endâmember is appropriately chosen. However, in most natural systems porewater exchange and groundwater inflow will occur concurrently. Our analysis shows that if groundwater inflow (Qg) and porewater exchange (Qp) mix completely before discharging to surface water, then the combined water flux (Qgâ+âQp) can be approximated by dividing the combined tracer flux by the difference between the porewater and surface water concentrations, (cp â c). If Qg and Qp do not mix prior to discharge, then (Qgâ+âQp) can only be constrained by minimum and maximum values. The minimum value is obtained by dividing the net tracer flux by the groundwater concentration, and the maximum is obtained by dividing by (cp â c). Dividing by the groundwater concentration gives a maximum value for Qg. If porewater exchange and groundwater outflow occur concurrently, then dividing the net tracer flux by (cp â c) will provide a minimum value for Qp. Use of multiple tracers, and spatial and temporal replication should provide a more complete picture of exchange processes and the extent of subsurface mixing
Shells around black holes: the effect of freely specifiable quantities in Einstein's constraint equations
We solve Einstein's constraint equations in the conformal thin-sandwich
decomposition to model thin shells of non-interacting particles in circular
orbit about a non-rotating black hole. We use these simple models to explore
the effects of some of the freely specifiable quantities in this decomposition
on the physical content of the solutions. Specifically, we adopt either maximal
slicing or Kerr-Schild slicing, and make different choices for the value of the
lapse on the black hole horizon. For one particular choice of these quantities
the resulting equations can be solved analytically; for all others we construct
numerical solutions. We find that these different choices have no effect on our
solutions when they are expressed in terms of gauge-invariant quantities.Comment: 9 pages, 4 figure
Can a combination of the conformal thin-sandwich and puncture methods yield binary black hole solutions in quasi-equilibrium?
We consider combining two important methods for constructing
quasi-equilibrium initial data for binary black holes: the conformal
thin-sandwich formalism and the puncture method. The former seeks to enforce
stationarity in the conformal three-metric and the latter attempts to avoid
internal boundaries, like minimal surfaces or apparent horizons. We show that
these two methods make partially conflicting requirements on the boundary
conditions that determine the time slices. In particular, it does not seem
possible to construct slices that are quasi-stationary and avoid physical
singularities and simultaneously are connected by an everywhere positive lapse
function, a condition which must obtain if internal boundaries are to be
avoided. Some relaxation of these conflicting requirements may yield a soluble
system, but some of the advantages that were sought in combining these
approaches will be lost.Comment: 8 pages, LaTeX2e, 2 postscript figure
HST Fine Guidance Sensor Astrometric Parallaxes for Three Dwarf Novae: SS Aurigae, SS Cygni, and U Geminorum
We report astrometric parallaxes for three well known dwarf novae obtained
using the Fine Guidance Sensors on the Hubble Space Telescope. We found a
parallax for SS Aurigae of Pi = 5.00 +/- 0.64 mas, for SS Cygni we found Pi =
6.02 +/- 0.46 mas, and for U Geminorum we obtained Pi = 10.37 +/- 0.50 mas.
These represent the first true trigonometric parallaxes of any dwarf novae. We
briefly compare these results with previous distance estimates. This program
demonstrates that with a very modest amount of HST observing time, the Fine
Guidance Sensors can deliver parallaxes of unrivaled precision.Comment: 15 pages, 2 Table
The Innermost Stable Circular Orbit of Binary Black Holes
We introduce a new method to construct solutions to the constraint equations
of general relativity describing binary black holes in quasicircular orbit.
Black hole pairs with arbitrary momenta can be constructed with a simple method
recently suggested by Brandt and Bruegmann, and quasicircular orbits can then
be found by locating a minimum in the binding energy along sequences of
constant horizon area. This approach produces binary black holes in a
"three-sheeted" manifold structure, as opposed to the "two-sheeted" structure
in the conformal-imaging approach adopted earlier by Cook. We focus on locating
the innermost stable circular orbit and compare with earlier calculations. Our
results confirm those of Cook and imply that the underlying manifold structure
has a very small effect on the location of the innermost stable circular orbit.Comment: 8 pages, 3 figures, RevTex, submitted to PR
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.
Quasiequilibrium sequences of black-hole--neutron-star binaries in general relativity
We construct quasiequilibrium sequences of black hole-neutron star binaries
for arbitrary mass ratios by solving the constraint equations of general
relativity in the conformal thin-sandwich decomposition. We model the neutron
star as a stationary polytrope satisfying the relativistic equations of
hydrodynamics, and account for the black hole by imposing equilibrium boundary
conditions on the surface of an excised sphere (the apparent horizon). In this
paper we focus on irrotational configurations, meaning that both the neutron
star and the black hole are approximately nonspinning in an inertial frame. We
present results for a binary with polytropic index n=1, mass ratio
M_{irr}^{BH}/M_{B}^{NS}=5 and neutron star compaction
M_{ADM,0}^{NS}/R_0=0.0879, where M_{irr}^{BH} is the irreducible mass of the
black hole, M_{B}^{NS} the neutron star baryon rest-mass, and M_{ADM,0}^{NS}
and R_0 the neutron star Arnowitt-Deser-Misner mass and areal radius in
isolation, respectively. Our models represent valid solutions to Einstein's
constraint equations and may therefore be employed as initial data for
dynamical simulations of black hole-neutron star binaries.Comment: 5 pages, 1 figure, revtex4, published in Phys.Rev.
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