1,491 research outputs found
Gravitational Waves from Neutron Stars with Large Toroidal B-fields
We show that NS's with large toroidal B-fields tend naturally to evolve into
potent gravitational-wave (gw) emitters. The toroidal field B_t tends to
distort the NS into a prolate shape, and this magnetic distortion can easily
dominate over the oblateness ``frozen into'' the NS crust. An elastic NS with
frozen-in B-field of this magnitude is clearly secularly unstable: the wobble
angle between the NS's angular momentum J^i and the star's magnetic axis n_B^i
grow on a dissipation timescale until J^i and n_B^i are orthogonal. This final
orientation is clearly the optimal one for gravitational-wave (gw) emission.
The basic cause of the instability is quite general, so we conjecture that the
same final state is reached for a realistic NS. Assuming this, we show that for
LMXB's with B_t of order 10^{13}G, the spindown from gw's is sufficient to
balance the accretion torque--supporting a suggestion by Bildsten. The spindown
rates of most millisecond pulsars can also be attributed to gw emission sourced
by toroidal B-fields, and both these sources could be observed by LIGO II.
While the first-year spindown of a newborn NS is most likely dominated by em
processes, reasonable values of B_t and the (external) dipolar field B_d can
lead to detectable levels of gw emission, for a newborn NS in our own galaxy.Comment: 7 pages; submitted to PRD; only minor revision
From essence to appearance: Parallels between the working methods of Alvar Aalto and Hugo Häring
The private house has perhaps always been a prototype for more elaborate architecture, but never more so than in the heyday of the Modern Movement, when it provided both the opportunity for experiment and the chance to explore creatively the changing essence of dwelling. This article, based on a keynote lecture given to the Alvar Aalto Foundation in February 2015, compares the generating principles of Alvar and Aino Aalto's own house of 1936 with those of built and unbuilt houses by Hugo Häring, underlining the importance of specific planning and spatial relationships as the essential generators, to which the making of façades remained subordinate or at least secondary. Häring's Woythaler House of 1927 also appears here for the first time in a properly comprehensible form, thanks to information recovered from drawings now available in the public realm
Superfluid Spin-down, with Random Unpinning of the Vortices
The so-called ``creeping'' motion of the pinned vortices in a rotating
superfluid involves ``random unpinning'' and ``vortex motion'' as two
physically separate processes. We argue that such a creeping motion of the
vortices need not be (biased) in the direction of an existing radial Magnus
force, nor should a constant microscopic radial velocity be assigned to the
vortex motion, in contradiction with the basic assumptions of the ``vortex
creep'' model. We point out internal inconsistencies in the predictions of this
model which arise due to this unjustified foundation that ignores the role of
the actual torque on the superfluid. The proper spin-down rate of a pinned
superfluid is then calculated and turns out to be much less than that suggested
in the vortex creep model, hence being of even less observational significance
for its possible application in explaining the post-glitch relaxations of the
radio pulsars.Comment: To be published in J. Low Temp. Phys., Vol. 139, May 2005 [Eqs 11,
15-17 here, have been revised and, may be substituted for the corresponding
ones in that paper
Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode
We study the dynamical evolution of a large amplitude r-mode by numerical
simulations. R-modes in neutron stars are unstable growing modes, driven by
gravitational radiation reaction. In these simulations, r-modes of amplitude
unity or above are destroyed by a catastrophic decay: A large amplitude r-mode
gradually leaks energy into other fluid modes, which in turn act nonlinearly
with the r-mode, leading to the onset of the rapid decay. As a result the
r-mode suddenly breaks down into a differentially rotating configuration. The
catastrophic decay does not appear to be related to shock waves at the star's
surface. The limit it imposes on the r-mode amplitude is significantly smaller
than that suggested by previous fully nonlinear numerical simulations.Comment: Published in Phys. Rev. D Rapid Comm. 66, 041303(R) (2002
Quantifying infective endocarditis risk in patients with predisposing cardiac conditions
Aims: There are scant comparative data quantifying the risk of infective endocarditis (IE) and associated mortality in individuals with predisposing cardiac conditions. Methods and results: English hospital admissions for conditions associated with increased IE risk were followed for 5 years to quantify subsequent IE admissions. The 5-year risk of IE or dying during an IE admission was calculated for each condition and compared with the entire English population as a control. Infective endocarditis incidence in the English population was 36.2/million/year. In comparison, patients with a previous history of IE had the highest risk of recurrence or dying during an IE admission [odds ratio (OR) 266 and 215, respectively]. These risks were also high in patients with prosthetic valves (OR 70 and 62) and previous valve repair (OR 77 and 60). Patients with congenital valve anomalies (currently considered 'moderate risk') had similar levels of risk (OR 66 and 57) and risks in other 'moderate-risk' conditions were not much lower. Congenital heart conditions (CHCs) repaired with prosthetic material (currently considered 'high risk' for 6 months following surgery) had lower risk than all 'moderate-risk' conditions-even in the first 6 months. Infective endocarditis risk was also significant in patients with cardiovascular implantable electronic devices. Conclusion: These data confirm the high IE risk of patients with a history of previous IE, valve replacement, or repair. However, IE risk in some 'moderate-risk' patients was similar to that of several 'high-risk' conditions and higher than repaired CHC. Guidelines for the risk stratification of conditions predisposing to IE may require re-evaluation
Conformal Field Theory Approach to the 2-Impurity Kondo Problem: Comparison with Numerical Renormalization Group Results
Numerical renormalization group and conformal field theory work indicate that
the two impurity Kondo Hamiltonian has a non-Fermi liquid critical point
separating the Kondo-screening phase from the inter-impurity singlet phase when
particle-hole (P-H) symmetry is maintained. We clarify the circumstances under
which this critical point occurs, pointing out that there are two types of P-H
symmetry. Only one of them guarantees the occurance of the critical point. Much
of the previous numerical work was done on models with the other type of P-H
symmetry. We analyse this critical point using the boundary conformal field
theory technique. The finite-size spectrum is presented in detail and compared
with about 50 energy levels obtained using the numerical renormalization group.
Various Green's functions, general renormalization group behaviour, and a
hidden are analysed.Comment: 38 pages, RevTex. 2 new sections clarify the circumstances under
which a model will exhibit the non-trivial critical point (hence potentially
resolving disagreements with other Authors) and explain the hidden SO(7)
symmetry of the model, relating it to an alternative approach of Sire et al.
and Ga
The cost-effectiveness of antibiotic prophylaxis for patients at risk of infective endocarditis
Background: In March 2008, the National Institute for Health and Care Excellence (NICE) recommended
stopping antibiotic prophylaxis (AP) for those at risk of infective endocarditis (IE) undergoing
dental procedures in the UK, citing a lack of evidence of efficacy and cost-effectiveness. We
have performed a new economic-evaluation of AP based on contemporary estimates of efficacy,
adverse events and resource implications.
Methods: A decision analytic cost-effectiveness model was used. Health service costs and benefits
(measured as Quality Adjusted Life Years, QALYs) were estimated. Rates of IE before and after
the NICE guidance were available to estimate prophylactic efficacy. AP adverse event rates were
derived from recent UK data and resource implications were based on English Hospital Episode
Statistics.
Results: AP was less costly and more effective than no AP for all patients at risk of IE. The results are
sensitive to AP efficacy, but efficacy would have to be substantially lower for AP not to be costeffective.
AP was even more cost-effective in patients at high-risk of IE. Only a marginal
reduction in annual IE rates (1.44 cases in high-risk and 33 cases in all at-risk patients) would be
required for AP to be considered cost-effective at £20,000 (7.3-10.9m) and health gains >2,600 QALYs could be achieved from
reinstating AP in England.
Conclusions: AP is cost-effective for preventing IE, particularly in those at high-risk. These findings support
the cost-effectiveness of guidelines recommending AP use in high-risk individuals
Geometric phases for non-degenerate and degenerate mixed states
This paper focuses on the geometric phase of general mixed states under
unitary evolution. Here we analyze both non-degenerate as well as degenerate
states. Starting with the non-degenerate case, we show that the usual procedure
of subtracting the dynamical phase from the total phase to yield the geometric
phase for pure states, does not hold for mixed states. To this end, we furnish
an expression for the geometric phase that is gauge invariant. The parallelity
conditions are shown to be easily derivable from this expression. We also
extend our formalism to states that exhibit degeneracies. Here with the
holonomy taking on a non-abelian character, we provide an expression for the
geometric phase that is manifestly gauge invariant. As in the case of the
non-degenerate case, the form also displays the parallelity conditions clearly.
Finally, we furnish explicit examples of the geometric phases for both the
non-degenerate as well as degenerate mixed states.Comment: 23 page
Depinning of a superfluid vortex line by Kelvin waves
We measure the interaction of a single superfluid vortex with surface
irregularities. While vortex pinning in superconductors usually becomes weaker
at higher temperatures, we find the opposite behavior. The pinning steadily
increases throughout our measurement range, from 0.15Tc to over 0.5Tc. We also
find that moving the other end of the vortex decreases the pinning, so we
propose Kelvin waves along the vortex as a depinning mechanism.Comment: 5 figures; substantial revision including 2 new figure
The r-modes in accreting neutron stars with magneto-viscous boundary layers
We explore the dynamics of the r-modes in accreting neutron stars in two
ways. First, we explore how dissipation in the magneto-viscous boundary layer
(MVBL) at the crust-core interface governs the damping of r-mode perturbations
in the fluid interior. Two models are considered: one assuming an
ordinary-fluid interior, the other taking the core to consist of superfluid
neutrons, type II superconducting protons, and normal electrons. We show,
within our approximations, that no solution to the magnetohydrodynamic
equations exists in the superfluid model when both the neutron and proton
vortices are pinned. However, if just one species of vortex is pinned, we can
find solutions. When the neutron vortices are pinned and the proton vortices
are unpinned there is much more dissipation than in the ordinary-fluid model,
unless the pinning is weak. When the proton vortices are pinned and the neutron
vortices are unpinned the dissipation is comparable or slightly less than that
for the ordinary-fluid model, even when the pinning is strong. We also find in
the superfluid model that relatively weak radial magnetic fields ~ 10^9 G (10^8
K / T)^2 greatly affect the MVBL, though the effects of mutual friction tend to
counteract the magnetic effects. Second, we evolve our two models in time,
accounting for accretion, and explore how the magnetic field strength, the
r-mode saturation amplitude, and the accretion rate affect the cyclic evolution
of these stars. If the r-modes control the spin cycles of accreting neutron
stars we find that magnetic fields can affect the clustering of the spin
frequencies of low mass x-ray binaries (LMXBs) and the fraction of these that
are currently emitting gravitational waves.Comment: 19 pages, 8 eps figures, RevTeX; corrected minor typos and added a
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