9,068 research outputs found
Double Charge Exchange And Configuration Mixing
The energy dependence of forward pion double charge exchange reactions on
light nuclei is studied for both the Ground State transition and the
Double-Isobaric-Analog-State transitions. A common characteristic of these
double reactions is a resonance-like peak around 50 MeV pion lab energy. This
peak arises naturally in a two-step process in the conventional pion-nucleon
system with proper handling of nuclear structure and pion distortion. A
comparison among the results of different nuclear structure models demonstrates
the effects of configuration mixing. The angular distribution is used to fix
the single particle wave function.Comment: Added 1 figure (now 8) corrected references and various other change
An X-Ray Study of the Supernova Remnant G290.1-0.8
G290.1-0.8 (MSH 11-61A) is a supernova remnant (SNR) whose X-ray morphology
is centrally bright. However, unlike the class of X-ray composite SNRs whose
centers are dominated by nonthermal emission, presumably driven by a central
pulsar, we show that the X-ray emission from G290.1-0.8 is thermal in nature,
placing the remnant in an emerging class which includes such remnants as W44,
W28, 3C391, and others. The evolutionary sequence which leads to such X-ray
properties is not well understood. Here we investigate two scenarios for such
emission: evolution in a cloudy interstellar medium, and early-stage evolution
of a remnant into the radiative phase, including the effects of thermal
conduction. We construct models for these scenarios in an attempt to reproduce
the observed center-filled X-ray properties of G290.1-0.8, and we derive the
associated age, energy, and ambient density conditions implied by the models.
We find that for reasonable values of the explosion energy, the remnant age is
of order (1 - 2) x 10^{4} yr. This places a fairly strong constraint on any
association between G290.1-0.8 and PSR J1105-610, which would require an
anomalously large velocity for the pulsar.Comment: 7 pages, 7 figures, ApJ, accepte
Amplitude Modulation and Relaxation-Oscillation of Counterpropagating Rolls within a Broken-Symmetry Laser-Induced Electroconvection Strip
We report a liquid-crystal pattern-formation experiment in which we break the
lateral (translational) symmetry of a nematic medium with a laser-induced
thermal gradient. The work is motivated by an improved measurement (reported
here) of the temperature dependence of the electroconvection threshold voltage
in planar-nematic 4-methoxybenzylidene-4-butylaniline (MBBA). In contrast with
other broken-symmetry-pattern studies that report a uniform drift, we observe a
strip of counterpropagating rolls that collide at a sink point, and a strong
temporally periodic amplitude modulation within a width of 3-4 rolls about the
sink point. The time dependence of the amplitude at a fixed position is
periodic but displays a nonsinusoidal relaxation-oscillation profile. After
reporting experimental results based on spacetime contours and wavenumber
profiles, along with a measurement of the change in the drift frequency with
applied voltage at a fixed control parameter, we propose some potential
guidelines for a theoretical model based on saddle-point solutions for
Eckhaus-unstable states and coupled complex Ginzburg-Landau equations.
Published in PRE 73, 036317 (2006).Comment: Published in Physical Review E in March 200
A novel mechanism of RNase L inhibition: Theiler\u27s virus L* protein prevents 2-5A from binding to RNase L
<div><p>The OAS/RNase L pathway is one of the best-characterized effector pathways of the IFN antiviral response. It inhibits the replication of many viruses and ultimately promotes apoptosis of infected cells, contributing to the control of virus spread. However, viruses have evolved a range of escape strategies that act against different steps in the pathway. Here we unraveled a novel escape strategy involving Theiler’s murine encephalomyelitis virus (TMEV) L* protein. Previously we found that L* was the first viral protein binding directly RNase L. Our current data show that L* binds the ankyrin repeats R1 and R2 of RNase L and inhibits 2’-5’ oligoadenylates (2-5A) binding to RNase L. Thereby, L* prevents dimerization and oligomerization of RNase L in response to 2-5A. Using chimeric mouse hepatitis virus (MHV) expressing TMEV L*, we showed that L* efficiently inhibits RNase L <i>in vivo</i>. Interestingly, those data show that L* can functionally substitute for the MHV-encoded phosphodiesterase ns2, which acts upstream of L* in the OAS/RNase L pathway, by degrading 2-5A.</p></div
De-biased Populations of Kuiper Belt Objects from the Deep Ecliptic Survey
The Deep Ecliptic Survey (DES) discovered hundreds of Kuiper Belt objects
from 1998-2005. Follow-up observations yielded 304 objects with good dynamical
classifications (Classical, Scattered, Centaur, or 16 mean-motion resonances
with Neptune). The DES search fields are well documented, enabling us to
calculate the probability of detecting objects with particular orbital
parameters and absolute magnitudes at a randomized point in each orbit.
Grouping objects together by dynamical class leads, we estimate the orbital
element distributions (a, e, i) for the largest three classes (Classical, 3:2,
and Scattered) using maximum likelihood. Using H-magnitude as a proxy for the
object size, we fit a power law to the number of objects for 8 classes with at
least 5 detected members (246 objects). The best Classical slope is
alpha=1.02+/-0.01 (observed from 5<=H<=7.2). Six dynamical classes (Scattered
plus 5 resonances) are consistent in slope with the Classicals, though the
absolute number of objects is scaled. The exception to the power law relation
are the Centaurs (non-resonant with perihelia closer than Neptune, and thus
detectable at smaller sizes), with alpha=0.42+/-0.02 (7.5<H<11). This is
consistent with a knee in the H-distribution around H=7.2 as reported elsewhere
(Bernstein et al. 2004, Fraser et al. 2014). Based on the Classical-derived
magnitude distribution, the total number of objects (H<=7) in each class are:
Classical (2100+/-300 objects), Scattered (2800+/-400), 3:2 (570+/-80), 2:1
(400+/-50), 5:2 (270+/-40), 7:4 (69+/-9), 5:3 (60+/-8). The independent
estimate for the number of Centaurs in the same H range is 13+/-5. If instead
all objects are divided by inclination into "Hot" and "Cold" populations,
following Fraser et al. (2014), we find that alphaHot=0.90+/-0.02, while
alphaCold=1.32+/-0.02, in good agreement with that work.Comment: 26 pages emulateapj, 6 figures, 5 tables, accepted by A
Phonon Assisted Multimagnon Optical Absorption and Long Lived Two-Magnon States in Undoped Lamellar Copper Oxides
We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in the parent insulating compounds of cuprate
superconductors and the spectra for two-magnon absorption using interacting
spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys.
Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a
two-magnon virtual bound state, and one phonon plus higher multimagnon
absorption processes. The virtual bound state consists of a narrow resonance
occurring when the magnon pair has total momentum close to .Comment: 4 page
Spectral Signatures of the Diffusional Anomaly in Water
Analysis of power spectrum profiles for various tagged particle quantities in
bulk SPC/E water is used to demonstrate that variations in mobility associated
with the diffusional anomaly are mirrored in the exponent of the \onebyf\
region. Monitoring of \onebyf behaviour is shown to be a simple and direct
method for linking phenomena on three distinctive length and time scales: the
local molecular environment, hydrogen bond network reorganisations and the
diffusivity. The results indicate that experimental studies of supercooled
water to probe the density dependence of spectral features, or
equivalent stretched exponential behaviour in time-correlation functions, will
be of interest.Comment: 5 Pages, 4 Figure
The Hypanis Valles delta: The last highstand of a sea on early Mars?
One of the most contentious hypotheses in the geological history of Mars is whether the northern lowlands ever contained an oceanic water body. Arguably, the best evidence for an ocean comes from the presence of sedimentary fans around Mars' dichotomy boundary, which separates the northern lowlands from the southern highlands. Here we describe the palaeogeomorphology of the Hypanis Valles sediment fan, the largest sediment fan complex reported on Mars (area >970 km2). This has an extensive catchment (4.6 x 105 km2) incorporating Hypanis and Nanedi Valles, that we show was active during the late-Noachian/early-Hesperian period (∼3.7 Ga). The fan comprises a series of lobe-shaped sediment bodies, connected by multiple bifurcating flat-topped ridges. We interpret the latter as former fluvial channel belts now preserved in inverted relief. Meter-scale-thick, sub-horizontal layers that are continuous over tens of kilometres are visible in scarps and the inverted channel margins. The inverted channel branches and lobes are observed to occur up to at least 140 km from the outlet of Hypanis Valles and descend ∼500 m in elevation. The progressive basinward advance of the channellobe transition records deposition and avulsion at the margin of a retreating standing body of water, assuming the elevation of the northern plains basin floor is stable. We interpret the Hypanis sediment fan to represent an ancient delta as opposed to a fluvial fan system. At its location at the dichotomy boundary, the Hypanis Valles fan system is topographically open to Chryse Planitia – an extensive plain that opens in turn into the larger northern lowlands basin. We conclude that the observed progradation of fan bodies was due to basinward shoreline retreat of an ancient body of water which extended across at least Chryse Planitia. Given the open topography, it is plausible that the Hypanis fan system records the existence, last highstand, and retreat of a large sea in Chryse Planitia and perhaps even an ocean that filled the northern plains of Mars
Systematic and Causal Corrections to the Coherent Potential Approximation
The Dynamical Cluster Approximation (DCA) is modified to include disorder.
The DCA incorporates non-local corrections to local approximations such as the
Coherent Potential Approximation (CPA) by mapping the lattice problem with
disorder, and in the thermodynamic limit, to a self-consistently embedded
finite-sized cluster problem. It satisfies all of the characteristics of a
successful cluster approximation. It is causal, preserves the point-group and
translational symmetry of the original lattice, recovers the CPA when the
cluster size equals one, and becomes exact as . We use the DCA to
study the Anderson model with binary diagonal disorder. It restores sharp
features and band tailing in the density of states which reflect correlations
in the local environment of each site. While the DCA does not describe the
localization transition, it does describe precursor effects of localization.Comment: 11 pages, LaTeX, and 11 PS figures, to appear in Phys. Rev. B.
Revised version with typos corrected and references adde
Electronic structure of triangular, hexagonal and round graphene flakes near the Fermi level
The electronic shell structure of triangular, hexagonal and round graphene
quantum dots (flakes) near the Fermi level has been studied using a
tight-binding method. The results show that close to the Fermi level the shell
structure of a triangular flake is that of free massless particles, and that
triangles with an armchair edge show an additional sequence of levels ("ghost
states"). These levels result from the graphene band structure and the plane
wave solution of the wave equation, and they are absent for triangles with an
zigzag edge. All zigzag triangles exhibit a prominent edge state at the Fermi
level, and few low-energy conduction electron states occur both in triangular
and hexagonal flakes due to symmetry reasons. Armchair triangles can be used as
building blocks for other types of flakes that support the ghost states. Edge
roughness has only a small effect on the level structure of the triangular
flakes, but the effect is considerably enhanced in the other types of flakes.
In round flakes, the states near the Fermi level depend strongly on the flake
radius, and they are always localized on the zigzag parts of the edge
- …