18,731 research outputs found
Diamagnetic Interactions in Disordered Suspensions of Metastable Superconducting Granules
The simulation of the transition sequence of superheated Type I
superconducting granules (SSG) in disordered suspensions when an external
magnetic field is slowly increased from zero has been studied. Simulation takes
into account diamagnetic interactions and the presence of surface defects.
Results have been obtained for the transition sequence and surface fields
distribution covering a wide range of densities. These results are compared
with previous analytical perturbative theory, which provides qualitative
information on transitions and surface magnetic fields during transitions, but
with a range of validity apparently limited to extremely dilute samples.
Simulations taking into account the complete diamagnetic interactions between
spheres appear to be a promising tool in interpreting SSG experiments, in
applications such as particle detectors, and in some fundamental calculations
of Solid State Physics.Comment: 25 pages, 11 figures, accepted for publication in European Physics
Journal
High-precision laser spectroscopy of the CO A - X (2,0), (3,0) and (4,0) bands
High-precision two-photon Doppler-free frequency measurements have been
performed on the CO A - X fourth-positive system (2,0),
(3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for
rotational quantum numbers up to , have been determined at an accuracy
of cm, using advanced techniques of two-color 2+1'
resonance-enhanced multi-photon ionization, Sagnac interferometry,
frequency-chirp analysis on the laser pulses, and correction for AC-Stark
shifts. The accurate transition frequencies of the CO A - X
system are of relevance for comparison with astronomical data in the search for
possible drifts of fundamental constants in the early universe. The present
accuracies in laboratory wavelengths of may be considered exact for the purpose of such comparisons.Comment: 13 pages, 6 figures, The Journal of Chemical Physics (2015) accepte
PAH Formation in O-rich Planetary Nebulae
Polycyclic aromatic hydrocarbons (PAHs) have been observed in O-rich
planetary nebulae towards the Galactic Bulge. This combination of oxygen-rich
and carbon-rich material, known as dual-dust or mixed chemistry, is not
expected to be seen around such objects. We recently proposed that PAHs could
be formed from the photodissociation of CO in dense tori. In this work, using
VISIR/VLT, we spatially resolved the emission of the PAH bands and ionised
emission from the [SIV] line, confirming the presence of dense central tori in
all the observed O-rich objects. Furthermore, we show that for most of the
objects, PAHs are located at the outer edge of these dense/compact tori, while
the ionised material is mostly present in the inner parts of these tori,
consistent with our hypothesis for the formation of PAHs in these systems. The
presence of a dense torus has been strongly associated with the action of a
central binary star and, as such, the rich chemistry seen in these regions may
also be related to the formation of exoplanets in post-common-envelope binary
systems.Comment: 14, accepted for publication in the MNRAS Journa
Semiclassical ordering in the large-N pyrochlore antiferromagnet
We study the semiclassical limit of the generalization of the
pyrochlore lattice Heisenberg antiferromagnet by expanding about the saddlepoint in powers of a generalized inverse spin. To leading order,
we write down an effective Hamiltonian as a series in loops on the lattice.
Using this as a formula for calculating the energy of any classical ground
state, we perform Monte-Carlo simulations and find a unique collinear ground
state. This state is not a ground state of linear spin-wave theory, and can
therefore not be a physical (N=1) semiclassical ground state.Comment: 4 pages, 4 eps figures; published versio
Effects of Platelet-Activating Factor on Brain Microvascular Endothelial Cells.
Platelet-activating factor (PAF) is a potent phospholipid mediator that exerts various pathophysiological effects by interacting with a G protein-coupled receptor. PAF has been reported to increase the permeability of the blood-brain barrier (BBB) via incompletely characterized mechanisms. We investigated the effect of PAF on rat brain microvascular endothelial cells (RBMVEC), a critical component of the BBB. PAF produced a dose-dependent increase in cytosolic Ca2+ concentration; the effect was prevented by the PAF receptor antagonist, WEB2086. The effect of PAF on cytosolic Ca2+ was abolished in Ca2+-free saline or in the presence of L-type voltage-gated Ca2+ channel inhibitor, nifedipine, indicating that Ca2+ influx is critical for PAF-induced increase in cytosolic Ca2+. PAF produced RBMVEC depolarization; the effect was inhibited by WEB2086. In cells loaded with [(4-amino-5-methylamino-2\u27,7\u27-difluoro-fluorescein)diacetate] (DAF-FM), a nitric oxide (NO)-sensitive fluorescent dye, PAF increased the NO level; the effect was prevented by WEB2086, nifedipine or by l-NAME, an inhibitor of NO synthase. Immunocytochemistry studies indicate that PAF reduced the immunostaining of ZO-1, a tight junction-associated protein, increased F-actin fibers, and produced intercellular gaps. PAF produced a decrease in RBMVEC monolayer electrical resistance assessed with Electric Cell-Substrate Impedance Sensing (ECIS), indicative of a disruption of endothelial barrier function. In vivo studies indicate that PAF increased the BBB permeability, assessed with sodium fluorescein and Evans Blue methods, via PAF receptor-dependent mechanisms, consequent to Ca2+ influx and increased NO levels. Our studies reveal that PAF alters the BBB permeability by multiple mechanisms, which may be relevant for central nervous system (CNS) inflammatory disorders
Disk evaporation in a planetary nebula
We study the Galactic bulge planetary nebula M 2-29 (for which a 3-year
eclipse event of the central star has been attributed to a dust disk) using HST
imaging and VLT spectroscopy, both long-slit and integral field. The central
cavity of M 2-29 is filled with a decreasing, slow wind. An inner high density
core is detected, with radius less than 250 AU, interpreted as a rotating
gas/dust disk with a bipolar disk wind. The evaporating disk is argued to be
the source of the slow wind. The central star is a source of a very fast wind
(1000 km/s). An outer, partial ring is seen in the equatorial plane, expanding
at 12 km/s. The azimuthal asymmetry is attributed to mass-loss modulation by an
eccentric binary. M 2-29 presents a crucial point in disk evolution, where
ionization causes the gas to be lost, leaving a low-mass dust disk behind.Comment: 11 pages, accepted for publication in "Astronomy and Astrophysics
A Possible Cepheid-Like Luminosity Estimator for the Long Gamma-Ray Bursts
We present a possible Cepheid-like luminosity estimator for the long
gamma-ray bursts based on the variability of their light curves. To construct
the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/KONUS data
for 5 bursts, Ulysses/GRB data for 1 burst, and NEAR/XGRS data for 1 burst.
Spectroscopic redshifts, peak fluxes, and high resolution light curves are
available for 11 of these bursts; partial information is available for the
remaining 9 bursts. We find that the isotropic-equivalent luminosities L of
these bursts positively correlate with a rigorously-constructed measure V of
the variability of their light curves. We fit a model to these data that
accommodates both intrinsic scatter (statistical variance) and extrinsic
scatter (sample variance). If one excludes GRB 980425 from the fit on the
grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not
secure, the luminosity estimator spans approx. 2.5 orders of magnitude in L,
and the slope of the correlation between L and V is positive with a probability
of 1 - 1.4 x 10^-4 (3.8 sigma). Although GRB 980425 is excluded from this fit,
its L and V values are consistent with the fitted model, which suggests that
GRB 980425 may well be associated with SN 1998bw, and that GRB 980425 and the
cosmological bursts may share a common physical origin. If one includes GRB
980425 in the fit, the luminosity estimator spans approx. 6.3 orders of
magnitude in L, and the slope of the correlation is positive with a probability
of 1 - 9.3 x 10^-7 (4.9 sigma). Independently of whether or not GRB 980425
should be included in the fit, its light curve is unique in that it is much
less variable than the other approx. 17 light curves in our sample for which
the signal-to-noise is reasonably good.Comment: Accepted to The Astrophysical Journal, 31 pages, 13 figures, LaTe
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