1,029 research outputs found
Observation of non-lithophile behavior for U
In the Hvittis enstatite chondrite fission track radiography shows that U is highly concentrated in CaS (oldhamite), with a concentration of 400±50 ppb. The oldhamite U concentrations are equilibrated, although intergrain concentration variations of less than 15% would not be detected. Various approaches to a U material balance do not agree, but at least 50% and, more likely, nearly 100% of the U is concentrated in CaS. It is likely that CaS is the major reservoir for other actinide and lanthanide elements as well. Consequently, enstatite chondrites may provide reliable Pu/U abundance ratios. The highly reducing formation conditions for enstatite chondrites have caused U to deviate from lithophile behaviour, but K is observed to remain lithophile. Although this observation may not be generalizable, we propose that discussions of radioactive heating of planetary cores should include U and Th and not focus exclusively on K
The influence of onium-substitution on the neuromuscular blocking properties of some quaternary ammonium compounds
Abstract Not Provided
The Radio Properties and Magnetic Field Configuration in the Crab-like Pulsar Wind Nebula G54.1+0.3
We present a multifrequency radio investigation of the Crab-like pulsar wind
nebula (PWN) G54.1+0.3 using the Very Large Array. The high resolution of the
observations reveals that G54.1+0.3 has a complex radio structure which
includes filamentary and loop-like structures that are magnetized, a diffuse
extent similar to the associated diffuse X-ray emission. But the radio and
X-ray structures in the central region differ strikingly, indicating that they
trace very different forms of particle injection from the pulsar and/or
particle acceleration in the nebula. No spectral index gradient is detected in
the radio emission across the PWN, whereas the X-ray emission softens outward
in the nebula. The extensive radio polarization allows us to image in detail
the intrinsic magnetic field, which is well-ordered and reveals that a number
of loop-like filaments are strongly magnetized. In addition, we determine that
there are both radial and toroidal components to the magnetic field structure
of the pulsar wind nebula. Strong mid-IR emission detected in Spitzer Space
Telescope data is closely correlated with the radio emission arising from the
southern edge of G54.1+0.3. In particular, the distributions of radio and X-ray
emission compared with the mid-IR emission suggest that the PWN may be
interacting with this interstellar cloud. This may be the first PWN where we
are directly detecting its interplay with an interstellar cloud that has
survived the impact of the supernova explosion associated with the pulsar's
progenitor.Comment: 35 pages, 19 figures, accepted for publication in the Astrophysical
Journa
X-Ray Observations of the supernova remnant G21.5-0.9
We present the analysis of archival X-ray observations of the supernova
remnant (SNR) G21.5-0.9. Based on its morphology and spectral properties,
G21.5-0.9 has been classified as a Crab-like SNR. In their early analysis of
the CHANDRA calibration data, Slane et al. (2000) discovered a
low-surface-brightness, extended emission. They interpreted this component as
the blast wave formed in the supernova (SN) explosion. In this paper, we
present the CHANDRA analysis using a total exposure of ~150 ksec. We also
include ROSAT and ASCA observations. Our analysis indicates that the extended
emission is non-thermal -- a result in agreement with XMM observations. The
entire remnant of radius ~ 2'.5 is best fitted with a power law model with a
photon index steepening away from the center. The total unabsorbed flux in the
0.5-10 keV is 1.1E-10 erg/cm2/s with an 85% contribution from the 40" radius
inner core. Timing analysis of the High-Resolution Camera (HRC) data failed to
detect any pulsations. We put a 16% upper limit on the pulsed fraction. We
derive the physical parameters of the putative pulsar and compare them with
those of other plerions (such as the Crab and 3C 58). G21.5-0.9 remains the
only plerion whose size in X-rays is bigger than in the radio. Deep radio
observations will address this puzzle.Comment: 23 pages including 11 figures and 3 tables; accepted by ApJ June 22,
2001; to appear in Oct 20, 2001 issue of Ap
Separating the low and high hierachies by oracles
AbstractThe relativized low and high hierarchies within NP are considered. An oracle A is constructed such that the low and high hierarchies relative to A are infinite, and for each k an oracle Ak is constructed such that the low and high hierarchies relative to Ak have exactly k levels
Structure formation in binary colloids
A theoretical study of the structure formation observed very recently [Phys.
Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model
solely the dipole-dipole interaction of the particles is considered,
electrohidrodynamic effects are excluded. Based on molecular dynamics
simulations and analytic calculations we show that the total concentration of
the particles, the relative concentration and the relative dipole moment of the
components determine the structure of the colloid. At low concentrations the
kinetic aggregation of particles results in fractal structures which show a
crossover behavior when increasing the concentration. At high concentration
various lattice structures are obtained in a good agreement with experiments.Comment: revtex, 4 pages, figures available from authors due to size problem
The Third VLBA Calibrator Survey
This paper presents the third extension to the Very Large Baseline Array
(VLBA) Calibrator Survey, containing 360 new sources not previously observed
with very long baseline interferometry (VLBI). The survey, based on three 24
hour VLBA observing sessions, fills the areas on the sky above declination -45
degrees where the calibrator density is less than one source within a 4 degrees
radius disk at any given direction. The positions were derived from astrometric
analysis of the group delays determined at 2.3 and 8.6 GHz frequency bands
using the Calc/Solve software package. The VCS3 catalogue of source positions,
plots of correlated flux density versus the length of projected baseline,
contour plots and fits files of naturally weighted CLEAN images as well as
calibrated visibility function files are available on the Web at
http://gemini.gsfc.nasa.gov/vcs3Comment: 8 pages, 3 figures, 2 tables, accepted for publication in the
Astronomical Journal; minor changes to the text are made, table 2 in
electronic form is added and can be extracted from the preprint sourc
Filamin C-related myopathies: pathology and mechanisms
The term filaminopathy was introduced after a truncating mutation in the dimerization domain of filamin C (FLNc) was shown to be responsible for a devastating muscle disease. Subsequently, the same mutation was found in patients from diverse ethnical origins, indicating that this specific alteration is a mutational hot spot. Patients initially present with proximal muscle weakness, while distal and respiratory muscles become affected with disease progression. Muscle biopsies of these patients show typical signs of myofibrillar myopathy, including disintegration of myofibrils and aggregation of several proteins into distinct intracellular deposits. Highly similar phenotypes were observed in patients with other mutations in Ig-like domains of FLNc that result in expression of a noxious protein. Biochemical and biophysical studies showed that the mutated domains acquire an abnormal structure causing decreased stability and eventually becoming a seed for abnormal aggregation with other proteins. The disease usually presents only after the fourth decade of life possibly as a result of ageing-related impairments in the machinery that is responsible for disposal of damaged proteins. This is confirmed by mutations in components of this machinery that cause a highly similar phenotype. Transfection studies of cultured muscle cells reflect the events observed in patient muscles and, therefore, may provide a helpful model for testing future dedicated therapeutic strategies. More recently, FLNC mutations were also found in families with a distal myopathy phenotype, caused either by mutations in the actin-binding domain of FLNc that result in increased actin-binding and non-specific myopathic abnormalities without myofibrillar myopathy pathology, or a nonsense mutation in the rod domain that leads to RNA instability, haploinsufficiency with decreased expression levels of FLNc in the muscle fibers and myofibrillar abnormalities, but not to the formation of desmin-positive protein aggregates required for the diagnosis of myofibrillar myopathy
Ergs: The Evolution of Shell Supernova Remnants
This paper reports on a workshop hosted by the University of Minnesota, March
23-26, 1997. It addressed fundamental dynamical issues associated with the
evolution of shell supernova remnants and the relationships between supernova
remnants and their environments. The workshop considered, in addition to
classical shell SNRs, dynamical issues involving X-ray filled composite
remnants and pulsar driven shells, such as that in the Crab Nebula.
Approximately 75 participants with wide ranging interests attended the
workshop. An even larger community helped through extensive on-line debates
prior to the meeting. Each of the several sessions, organized mostly around
chronological labels, also addressed some underlying, general physical themes:
How are SNR dynamics and structures modified by the character of the CSM and
the ISM and vice versa? How are magnetic fields generated in SNRs and how do
magnetic fields influence SNRs? Where and how are cosmic-rays (electrons and
ions) produced in SNRs and how does their presence influence or reveal SNR
dynamics? How does SNR blast energy partition into various components over time
and what controls conversion between components? In lieu of a proceedings
volume, we present here a synopsis of the workshop in the form of brief
summaries of the workshop sessions. The sharpest impressions from the workshop
were the crucial and under-appreciated roles that environments have on SNR
appearance and dynamics and the critical need for broad-based studies to
understand these beautiful, but enigmatic objects. \\Comment: 54 pages text, no figures, Latex (aasms4.sty). submitted to the PAS
Re-Architecting the NASA Wire Derating Approach for Space Flight Applications
Mr. Steve Rickman, NASA Technical Fellow for Passive Thermal, proposed a pathfinder study to develop an apparatus for wire and wire bundle thermal testing to measure their performance, and to support development of thermal analytical models. Development of such capability would enable wire and wire bundle amperage capacity. The goal of this study was to assess the feasibility of developing physics-based and regression thermal models of single wires and wire bundles. This report contains the outcome of the NESC assessment
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