91,237 research outputs found
The stability of new transparent polymeric materials: The epoxy trimethoxyboroxine system. Part 1: The preparation, characterization and curing of epoxy resins and their copolymers
The effects of resin composition, curing conditions fillers, and flame retardant additives on the flammability of diglycidyl ether of bisphenol-A (DGEBA) as measured by the oxygen index is examined. The oxygen index of DGEBA cured with various curing agents was between 0.198 to 0.238. Fillers and flame retardant additives can increase the oxygen index dependent on the material and the amount used. Changes in the basic cured resin properties can be anticipated with the addition of noncompatible additives. High flame resistant epoxy resins with good stability and mechanical properties are investigated
c(2x2) Interface Alloys in Co/Cu Multilayers - Influence on Interlayer Exchange Coupling and GMR
The influence of a c(2x2) ordered interface alloy of 3d transition metals at
the ferromagnet/nonmagnet interface on interlayer exchange coupling (IXC), the
formation of quantum well states (QWS) and the phenomenon of Giant
MagnetoResistance is investigated. We obtained a strong dependence of IXC on
interface alloy formation. The GMR ratio is also strongly influenced. We found
that Fe, Ni and Cu alloys at the interface enhance the GMR ratio for in-plane
geometry by nearly a factor of 2.Comment: 14 pages, 5 figures, 1 table, subm. to PR
Deep-inelastic scattering and the operator product expansion in lattice QCD
We discuss the determination of deep-inelastic hadron structure in lattice
QCD. By using a fictitious heavy quark, direct calculations of the Compton
scattering tensor can be performed in Euclidean space that allow the extraction
of the moments of structure functions. This overcomes issues of operator mixing
and renormalisation that have so far prohibited lattice computations of higher
moments. This approach is especially suitable for the study of the twist-two
contributions to isovector quark distributions, which is practical with current
computing resources. While we focus on the isovector unpolarised distribution,
our method is equally applicable to other quark distributions and to
generalised parton distributions. By looking at matrix elements such as
(where and are vector
and axial-vector heavy-light currents) within the same formalism, moments of
meson distribution amplitudes can also be extracted.Comment: 10 pages, 5 figures, version accepted for publicatio
A negative mass theorem for surfaces of positive genus
We define the "sum of squares of the wavelengths" of a Riemannian surface
(M,g) to be the regularized trace of the inverse of the Laplacian. We normalize
by scaling and adding a constant, to obtain a "mass", which is scale invariant
and vanishes at the round sphere. This is an anlaog for closed surfaces of the
ADM mass from general relativity. We show that if M has positive genus then on
each conformal class, the mass attains a negative minimum. For the minimizing
metric, there is a sharp logarithmic Hardy-Littlewood-Sobolev inequality and a
Moser-Trudinger-Onofri type inequality.Comment: 8 page
Three-region inequalities for the second order elliptic equation with discontinuous coefficients and size estimate
In this paper, we would like to derive a quantitative uniqueness estimate,
the three-region inequality, for the second order elliptic equation with jump
discontinuous coefficients. The derivation of the inequality relies on the
Carleman estimate proved in our previous work. We then apply the three-region
inequality to study the size estimate problem with one boundary measurement.Comment: 16 pages, 1 figur
CD4+ T-cell responses to Epstein-Barr virus nuclear antigen EBNA1 in Chinese populations are highly focused on novel C-terminal domain-derived epitopes
Epstein-Barr virus nuclear antigen EBNA1, the one viral protein uniformly expressed in nasopharyngeal carcinoma (NPC), represents a prime target for T-cell-based immunotherapy. However, little is known about the EBNA1 epitopes, particularly CD4 epitopes, presented by HLA alleles in Chinese people, the group at highest risk for NPC. We analyzed the CD4 T-cell responses to EBNA1 in 78 healthy Chinese donors and found marked focusing on a small number of epitopes in the EBNA1 C-terminal region, including a DP5- restricted epitope that was recognized by almost half of the donors tested and elicited responses able to recognize EBNA1-expressing, DP5-positive target cells
The Giant Flare of December 27, 2004 from SGR 1806-20
The giant flare of December 27, 2004 from SGR 1806-20 represents one of the
most extraordinary events captured in over three decades of monitoring the
gamma-ray sky. One measure of the intensity of the main peak is its effect on
X- and gamma-ray instruments. RHESSI, an instrument designed to study the
brightest solar flares, was completely saturated for ~0.5 s following the start
of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the
time of the giant flare, however, allowed RHESSI a unique view of the giant
flare event, including the precursor, the main peak decay, and the pulsed tail.
Since RHESSI was saturated during the main peak, we augment these observations
with Wind and RHESSI particle detector data in order to reconstruct the main
peak as well. Here we present detailed spectral analysis and evolution of the
giant flare. We report the novel detection of a relatively soft fast peak just
milliseconds before the main peak, whose timescale and sizescale indicate a
magnetospheric origin. We present the novel detection of emission extending up
to 17 MeV immediately following the main peak, perhaps revealing a
highly-extended corona driven by the hyper-Eddington luminosities. The spectral
evolution and pulse evolution during the tail are presented, demonstrating
significant magnetospheric twist and evolution during this phase. Blackbody
radii are derived for every stage of the flare, which show remarkable agreement
despite the range of luminosities and temperatures covered. Finally, we place
significant upper limits on afterglow emission in the hundreds of seconds
following the giant flare.Comment: 32 pages, 14 figures, submitted to Ap
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
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