40,813 research outputs found
BFT Hamiltonian embedding for SU(3) Skyrmion
We newly apply the Batalin, Fradkin and Tyutin (BFT) formalism to the SU(3)
flavor Skyrmion model to investigate the Weyl ordering correction to the
structure of the hyperfine splittings of strange baryons. On the other hand,
the Berry phases and Casimir effects are also discussed.Comment: 14 pages, modified titl
Late-Time Convection in the Collapse of a 23 Solar Mass Star
The results of a 3-dimensional SNSPH simulation of the core collapse of a 23
solar mass star are presented. This simulation did not launch an explosion
until over 600ms after collapse, allowing an ideal opportunity to study the
evolution and structure of the convection below the accretion shock to late
times. This late-time convection allows us to study several of the recent
claims in the literature about the role of convection: is it dominated by an
l=1 mode driven by vortical-acoustic (or other) instability, does it produce
strong neutron star kicks, and, finally, is it the key to a new explosion
mechanism? The convective region buffets the neutron star, imparting a 150-200
km/s kick. Because the l=1 mode does not dominate the convection, the neutron
star does not achieve large (>450 km/s) velocities. Finally, the neutron star
in this simulation moves, but does not develop strong oscillations, the energy
source for a recently proposed supernova engine. We discuss the implications
these results have on supernovae, hypernovae (and gamma-ray bursts), and
stellar-massed black holes.Comment: 31 pages (including 13 figures), submitted to Ap
Nonreactive mixing study of a scramjet swept-strut fuel injector
The results are presented of a cold-mixing investigation performed to supply combustor design information and to determine optimum normal fuel-injector configurations for a general scramjet swept-strut fuel injector. The experimental investigation was made with two swept struts in a closed duct at a Mach number of 4.4 and a nominal ratio of jet mass flow to air mass flow of 0.0295, with helium used to simulate hydrogen fuel. Four injector patterns were evaluated; they represented the range of hole spacing and the ratio of jet dynamic pressure to free-stream dynamic pressure. Helium concentration, pitot pressure, and static pressure in the downstream mixing region were measured to generate the contour plots needed to define the mixing-region flow field and the mixing parameters. Experimental results show that the fuel penetration from the struts was less than the predicted values based on flat-plate data; but the mixing rate was faster and produced a mixing length less than one-half that predicted
Graphene field-effect transistors based on boron nitride gate dielectrics
Graphene field-effect transistors are fabricated utilizing single-crystal
hexagonal boron nitride (h-BN), an insulating isomorph of graphene, as the gate
dielectric. The devices exhibit mobility values exceeding 10,000 cm2/V-sec and
current saturation down to 500 nm channel lengths with intrinsic
transconductance values above 400 mS/mm. The work demonstrates the favorable
properties of using h-BN as a gate dielectric for graphene FETs.Comment: 4 pages, 8 figure
UCN Upscattering rates in a molecular deuterium crystal
A calculation of ultra-cold neutron (UCN) upscattering rates in molecular
deuterium solids has been carried out, taking into account intra-molecular
exictations and phonons. The different moelcular species ortho-D2 (with even
rotational quantum number J) and para-D2 (with odd J) exhibit significantly
different UCN-phonon annihilation cross-sections. Para- to ortho-D2 conversion,
furthermore, couples UCN to an energy bath of excited rotational states without
mediating phonons. This anomalous upscattering mechanism restricts the UCN
lifetime to 4.6 msec in a normal-D2 solid with 33% para content.Comment: 3 pages, one figur
Numerical Study of Wave Propagation in Uniaxially Anisotropic Lorentzian Backward Wave Slabs
The propagation and refraction of a cylindrical wave created by a line
current through a slab of backward wave medium, also called left-handed medium,
is numerically studied with FDTD. The slab is assumed to be uniaxially
anisotropic. Several sets of constitutive parameters are considered and
comparisons with theoretical results are made. Electric field distributions are
studied inside and behind the slab. It is found that the shape of the
wavefronts and the regions of real and complex wave vectors are in agreement
with theoretical results.Comment: 6 pages, figure
X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation Mapping
One of the principal scientific objectives of the upcoming Constellation-X
mission is to attempt to map the inner regions of accretion disks around black
holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence
line. This area of the disk is likely radiation pressure dominated and subject
to various dynamical instabilities. Here, we show that density inhomogeneities
in the disk atmosphere resulting from the photon bubble instability (PBI) can
cause rapid changes in the X-ray reflection features, even when the
illuminating flux is constant. Using a simulation of the development of the
PBI, we find that, for the disk parameters chosen, the Fe K and O VIII Ly\alpha
lines vary on timescales as short as a few hundredths of an orbital time. In
response to the changes in accretion disk structure, the Fe K equivalent width
(EW) shows variations as large as ~100 eV. The magnitude and direction
(positive or negative) of the changes depends on the ionization state of the
atmosphere. The largest changes are found when the disk is moderately ionized.
The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid,
low-amplitude changes. This effect provides a natural explanation for some
observed instances of short timescale Fe K variability which was uncorrelated
with the continuum (e.g., Mrk 841). New predictions for Fe K reverberation
mapping should be made which include the effects of this accretion disk driven
line variability and a variable ionization state. Reflection spectra averaged
over the evolution of the instability are well fit by constant density models
in the 2-10 keV region.Comment: 20 pages, 3 figures. Accepted by Ap
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