874 research outputs found
Meteorites on Mars observed with the Mars Exploration Rovers
Reduced weathering rates due to the lack of liquid water and significantly greater typical surface ages should result in a higher density of meteorites on the surface of Mars compared to Earth. Several meteorites were identified among the rocks investigated during Opportunity’s traverse across the sandy Meridiani plains. Heat Shield Rock is a IAB iron meteorite and has been officially recognized as ‘‘Meridiani Planum.’’ Barberton is olivine-rich and contains metallic Fe in the form of kamacite, suggesting a meteoritic origin. It is chemically most consistent with a mesosiderite silicate clast. Santa Catarina is a brecciated rock with a chemical and mineralogical composition similar to Barberton. Barberton, Santa Catarina, and cobbles adjacent to Santa Catarina may be part of a strewn field. Spirit observed two probable iron meteorites from its Winter Haven location in the Columbia Hills in Gusev Crater. Chondrites have not been identified to date, which may be a result of their lower strengths and probability to survive impact at current atmospheric pressures. Impact craters directly associated with Heat Shield Rock, Barberton, or Santa Catarina have not been observed, but such craters could have been erased by eolian-driven erosion.Additional co-authors: DW Ming, RV Morris, PA de Souza Jr, SW Squyres, C Weitz, AS Yen, J Zipfel, T Economo
Quantum solitons at strong coupling
We examine the effect of one loop quantum corrections on the formation of
nontopological solitons in a strongly coupled scalar-fermionic Yukawa theory.
The exact one fermion loop contribution is incorporated by using a nonlocal
method to correct the local derivative expansion approximation (DE) of the
effective action. As the Yukawa coupling is increased we find that the nonlocal
corrections play an increasingly important role. The corrections cause the
scalar field to increase in depth while maintaining its size. This increases
the energy of the bag configuration, but this is compensated for by more
tightly bound fermionic states with lower energy. In contrast to the
semi-classical picture without quantum corrections, the binding energy is
small, and the total energy scales directly with the Yukawa coupling. This
confirms the qualitative behavior found in earlier work using the second order
DE, although the quantitative solutions differ.Comment: 17 pages, 4 Postscript figures, REVTe
Search for Magnetic Monopoles Trapped in Matter
There have been many searches for magnetic monopoles in flight, but few for
monopoles in matter. We have searched for magnetic monopoles in meteorites,
schists, ferromanganese nodules, iron ores and other materials. The detector
was a superconducting induction coil connected to a SQUID (Superconducting
Quantum Interference Device) with a room temperature bore 15 cm in diameter. We
tested a total of more than 331 kg of material including 112 kg of meteorites.
We found no monopole and conclude the overall monopole/nucleon ratio in the
samples is with a 90\% confidence level.Comment: 6 pages, rev tex, no figure
Quantum Kinks: Solitons at Strong Coupling
We examine solitons in theories with heavy fermions. These ``quantum''
solitons differ dramatically from semi-classical (perturbative) solitons
because fermion loop effects are important when the Yukawa coupling is strong.
We focus on kinks in a --dimensional theory coupled to
fermions; a large- expansion is employed to treat the Yukawa coupling
nonperturbatively. A local expression for the fermion vacuum energy is derived
using the WKB approximation for the Dirac eigenvalues. We find that fermion
loop corrections increase the energy of the kink and (for large ) decrease
its size. For large , the energy of the quantum kink is proportional to ,
and its size scales as , unlike the classical kink; we argue that these
features are generic to quantum solitons in theories with strong Yukawa
couplings. We also discuss the possible instability of fermions to solitons.Comment: 21 pp. + 2 figs., phyzzx, JHU-TIPAC-92001
Theoretical Evaluations of the Fission Cross Section of the 77 eV Isomer of 235-U
We have developed models of the fission barrier (barrier heights and
transition state spectra) that reproduce reasonably well the measured fission
cross section of U from neutron energy of 1 keV to 2 MeV. From these
models we have calculated the fission cross section of the 77 eV isomer of
U over the same energy range. We find that the ratio of the isomer
cross section to that of the ground state lies between about 0.45 and 0.55 at
low neutron energies. The cross sections become approximately equal above 1
MeV. The ratio of the neutron capture cross section to the fission cross
section for the isomer is predicted to be about a factor of 3 larger for the
isomer than for the ground state of U at keV neutron energies. We have
also calculated the cross section for the population of the isomer by inelastic
neutron scattering form the U ground state. We find that the isomer is
strongly populated, and for the cross section
leading to the population of the isomer is of the order of 0.5 barn. Thus,
neutron reaction network calculations involving the uranium isotopes in a high
neutron fluence are likely to be affected by the 77 eV isomer of U.
With these same models the fission cross sections of U and U
can be reproduced approximately using only minor adjustments to the barrier
heights. With the significant lowering of the outer barrier that is expected
for the outer barrier the general behavior of the fission cross section of
Pu can also be reproduced.Comment: 17 pages including 8 figure
Selected Papers on Protoplanetary Disks
Three papers present studies of thermal balances, dynamics, and electromagnetic spectra of protoplanetary disks, which comprise gas and dust orbiting young stars. One paper addresses the reprocessing, in a disk, of photons that originate in the disk itself in addition to photons that originate in the stellar object at the center. The shape of the disk is found to strongly affect the redistribution of energy. Another of the three papers reviews an increase in the optical luminosity of the young star FU Orionis. The increase began in the year 1936 and similar increases have since been observed in other stars. The paper summarizes astronomical, meteoric, and theoretical evidence that these increases are caused by increases in mass fluxes through the inner portions of the protoplanetary disks of these stars. The remaining paper presents a mathematical-modeling study of the structures of protostellar accretion disks, with emphasis on limits on disk flaring. Among the conclusions reached in the study are that (1) the radius at which a disk becomes shadowed from its central stellar object depends on radial mass flow and (2) most planet formation has occurred in environments unheated by stellar radiation
Physical Activity and the Health of Wheelchair Users: A Systematic Review in Multiple Sclerosis, Cerebral Palsy, and Spinal Cord Injury
Objective
To understand the benefits and harms of physical activity in people who may require a wheelchair with a focus on people with multiple sclerosis (MS), cerebral palsy (CP), and spinal cord injury (SCI).
Data Sources
Searches were conducted in MEDLINE, Cumulative Index to Nursing and Allied Health, PsycINFO, Cochrane CENTRAL, and Embase (January 2008 through November 2020).
Study Selection
Randomized controlled trials, nonrandomized trials, and cohort studies of observed physical activity (at least 10 sessions on 10 days) in participants with MS, CP, and SCI.
Data Extraction
We conducted dual data abstraction, quality assessment, and strength of evidence. Measures of physical functioning are reported individually where sufficient data exist and grouped as “function” where data are scant.
Data Synthesis
No studies provided evidence for prevention of cardiovascular conditions, development of diabetes, or obesity. Among 168 included studies, 44% enrolled participants with MS (38% CP, 18% SCI). Studies in MS found walking ability may be improved with treadmill training and multimodal exercises; function may be improved with treadmill, balance exercises, and motion gaming; balance is likely improved with balance exercises and may be improved with aquatic exercises, robot-assisted gait training (RAGT), motion gaming, and multimodal exercises; activities of daily living (ADL), female sexual function, and spasticity may be improved with aquatic therapy; sleep may be improved with aerobic exercises and aerobic fitness with multimodal exercises. In CP, balance may be improved with hippotherapy and motion gaming; function may be improved with cycling, treadmill, and hippotherapy. In SCI, ADL may be improved with RAGT.
Conclusions
Depending on population and type of exercise, physical activity was associated with improvements in walking, function, balance, depression, sleep, ADL, spasticity, female sexual function, and aerobic capacity. Few harms of physical activity were reported in studies. Future studies are needed to address evidence gaps and to confirm findings
Systematic chemical variations in large 3AB iron meteorites: Clues to core crystallization
Analysis of numerous individual iron meteorites have shown that fractional crystallization of iron cores result in variations in chemical concentration of the solid core which span several orders of magnitude. The magnitude and direction of the resulting spatial gradients in the core can provide clues to the physical nature of the core crystallization process. We have analyzed suites of samples from three large 3AB irons (Cape York, 58t; Chupaderos, 24t; Morito, 10t) in order to estimate local chemical gradients. Initial results for the concentrations of Ge, Pd, Pt (Massey group), Ir, Au, As, Co, Os, and Rh (Dalhouse group), and P (Arizona group) show significant ranges among the Cape York and Chupaderos samples and marginally significant ranges among the Morito samples. Measurements of Au, Ir, Co, Ni, Cu, Ga, As, W, Re (from UCLA) and Ni and Co (Arizona group) are in progress. We find a spatial Ir gradient in Chupaderos with a magnitude similar to the one reported for Agpalilik (Cape York iron) by Esbensen et al
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