18,362 research outputs found
Spray-on technique simplifies fabrication of complex thermal insulation blanket
Spray-on process constructs molds used in forming sections of thermal insulation blankets. The process simplifies the fabrication of blankets by eliminating much of the equipment formerly required and decreasing the time involved
Interactions of vortices with rarefaction solitary waves in a Bose-Einstein condensate and their role in the decay of superfluid turbulence
There are several ways to create the vorticity-free solitary waves --
rarefaction pulses -- in condensates: by the process of strongly nonequilibrium
condensate formation in a weakly interacting Bose gas, by creating local
depletion of the condensate density by a laser beam, and by moving a small
object with supercritical velocities. Perturbations created by such waves
colliding with vortices are studied in the context of the Gross-Pitaevskii
model. We find that the effect of the interactions consists of two competing
mechanisms: the creation of vortex line as rarefaction waves acquire
circulation in a vicinity of a vortex core and the loss of the vortex line to
sound due to Kelvin waves that are generated on vortex lines by rarefaction
pulses. When a vortex ring collides with a rarefaction wave, the ring either
stabilises to a smaller ring after emitting sound through Kelvin wave radiation
or the entire energy of the vortex ring is lost to sound if the radius of the
ring is of the order of the healing length. We show that during the time
evolution of a tangle of vortices, the interactions with rarefaction pulses
provide an important dissipation mechanism enhancing the decay of superfluid
turbulence.Comment: Revised paper accepted by Phys. Rev.
Testing the cooling flow model in the intermediate polar EX Hydrae
We use the best available X-ray data from the intermediate polar EX Hydrae to
study the cooling-flow model often applied to interpret the X-ray spectra of
these accreting magnetic white dwarf binaries. First, we resolve a
long-standing discrepancy between the X-ray and optical determinations of the
mass of the white dwarf in EX Hya by applying new models of the inner disk
truncation radius. Our fits to the X-ray spectrum now agree with the white
dwarf mass of 0.79 Msun determined using dynamical methods through
spectroscopic observations of the secondary. We use a simple isobaric cooling
flow model to derive the emission line fluxes, emission measure distribution,
and H-like to He-like line ratios for comparison with the 496 ks Chandra High
Energy Transmission Grating observation of EX Hydrae. We find that the H/He
ratios are not well reproduced by this simple isobaric cooling flow model and
show that while H-like line fluxes can be accurately predicted, fluxes of
lower-Z He-like lines are significantly underestimated. This discrepancy
suggests that some extra heating mechanism plays an important role at the base
of the accretion column, where cooler ions form. We thus explored more complex
cooling models including the change of gravitational potential with height in
the accretion column and a magnetic dipole geometry. None of these
modifications to the standard cooling flow model are able to reproduce the
observed line ratios. While a cooling flow model with subsolar (0.1 )
abundances is able to reproduce the line ratios by reducing the cooling rate at
temperatures lower than K, the predicted line-to-continuum
ratios are much lower than observed. We discuss and discard mechanisms such as
photoionization, departures from constant pressure, resonant scattering,
different electron-ion temperatures, and Compton cooling. [Abridged]Comment: Accepted in Astronomy & Astrophysics, modified version after referee
comments and proof correction
Molecular dynamics study of solvation effects on acid dissociation in aprotic media
Acid ionization in aprotic media is studied using Molecular Dynamics
techniques. In particular, models for HCl ionization in acetonitrile and
dimethylsulfoxide are investigated. The proton is treated quantum mechanically
using Feynman path integral methods and the remaining molecules are treated
classically. Quantum effects are shown to be essential for the proper treatment
of the ionization. The potential of mean force is computed as a function of the
ion pair separation and the local solvent structure is examined. The computed
dissociation constants in both solvents differ by several orders of magnitude
which are in reasonable agreement with experimental results. Solvent separated
ion pairs are found to exist in dimethylsulfoxide but not in acetonitrile.
Dissociation mechanisms in small clusters are also investigated. Solvent
separated ion pairs persist even in aggregates composed of rather few
molecules, for instance, as few as thirty molecules. For smaller clusters or
for large ion pair separations cluster finite-size effects come into play in a
significant fashion.Comment: Plain LaTeX. To appear in JCP(March 15). Mpeg simulations available
at http://www.chem.utoronto.ca/staff/REK/Videos/clusters/clusters.htm
Photoionized features in the X-ray spectrum of Ex Hydrae
We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae ( EX Hya). In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely, O VIII lambda 18.97, Mg XII lambda 8.42, Si XIV lambda 6.18, and Fe XVII lambda 16.78. The broad and narrow components have widths of similar to 1600 km s(-1) and similar to 150 km s(-1), respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions such as O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables
Navigation/traffic control satellite mission study. Volume 2 - Systems analyses
Systems analysis of spacecraft network for transoceanic traffic contro
Non-linear exciton spin-splitting in single InAs/GaAs self-assembled quantum structures in ultrahigh magnetic fields
We report on the magnetic field dispersion of the exciton spin-splitting and
diamagnetic shift in single InAs/GaAs quantum dots (QDs) and dot molecules
(QDMs) up to = 28 T. Only for systems with strong geometric confinement,
the dispersions can be well described by simple field dependencies, while for
dots with weaker confinement considerable deviations are observed: most
importantly, in the high field limit the spin-splitting shows a non-linear
dependence on , clearly indicating light hole admixtures to the valence band
ground state
Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of
high-resolution X-ray spectra in which hydrogen- and helium-like ions are
usually strong features. We present results from a new collisional-radiative
plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic
data in the companion Astrophysical Plasma Emission Database (APED) to
calculate spectral models for hot plasmas. APED contains the requisite atomic
data such as collisional and radiative rates, recombination cross sections,
dielectronic recombination rates, and satellite line wavelengths. We compare
the APEC results to other plasma codes for hydrogen- and helium-like
diagnostics, and test the sensitivity of our results to the number of levels
included in the models. We find that dielectronic recombination with
hydrogen-like ions into high (n=6-10) principal quantum numbers affects some
helium-like line ratios from low-lying (n=2) transitions.Comment: 5 pages, 6 figures, accepted by ApJ Letter
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