10,552 research outputs found
Stellar Chemical Abundances: In Pursuit of the Highest Achievable Precision
The achievable level of precision on photospheric abundances of stars is a
major limiting factor on investigations of exoplanet host star characteristics,
the chemical histories of star clusters, and the evolution of the Milky Way and
other galaxies. While model-induced errors can be minimized through the
differential analysis of spectrally similar stars, the maximum achievable
precision of this technique has been debated. As a test, we derive differential
abundances of 19 elements from high-quality asteroid-reflected solar spectra
taken using a variety of instruments and conditions. We treat the solar spectra
as being from unknown stars and use the resulting differential abundances,
which are expected to be zero, as a diagnostic of the error in our
measurements. Our results indicate that the relative resolution of the target
and reference spectra is a major consideration, with use of different
instruments to obtain the two spectra leading to errors up to 0.04 dex. Use of
the same instrument at different epochs for the two spectra has a much smaller
effect (~0.007 dex). The asteroid used to obtain the solar standard also has a
negligible effect (~0.006 dex). Assuming that systematic errors from the
stellar model atmospheres have been minimized, as in the case of solar twins,
we confirm that differential chemical abundances can be obtained at sub-0.01
dex precision with due care in the observations, data reduction and abundance
analysis.Comment: Accepted for publication in ApJ; 13 pages, 6 figures, 7 table
On the geometry of Riemannian cubic polynomials
AbstractWe continue the work of Crouch and Silva Leite on the geometry of cubic polynomials on Riemannian manifolds. In particular, we generalize the theory of Jacobi fields and conjugate points and present necessary and sufficient optimality condition
A Nuclear Physics Program at the ATLAS Experiment at the CERN Large Hadron Collider
The ATLAS collaboration has significant interest in the physics of
ultra-relativistic heavy ion collisions. We submitted a Letter of Intent to the
United States Department of Energy in March 2002. The following document is a
slightly modified version of that LOI. More details are available at:
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/SM/ionsComment: Letter of Intent submitted to the United States Department of Energy
Nuclear Physics Division in March 2002 (revised version
Uhlenbeck-Ford model: phase diagram and corresponding-states analysis
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESUsing molecular dynamics simulations and nonequilibrium thermodynamic-integration techniques we compute the Helmholtz free energies of the body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal close-packed, and fluid phases of the Uhlenbeck-Ford model (UFM) and use the results to construct its phase diagram. The pair interaction associated with the UFM is characterized by an ultrasoft, purely repulsive pair potential that diverges logarithmically at the origin. We find that the bcc and fcc are the only thermodynamically stable crystalline phases in the phase diagram. Furthermore, we report the existence of two reentrant transition sequences as a function of the number density, one featuring a fluid-bcc-fluid succession and another displaying a bcc-fcc-bcc sequence near the triple point. We find strong resemblances to the phase behavior of other soft, purely repulsive systems such as the Gaussian-core model (GCM), inverse-power-law, and Yukawa potentials. In particular, we find that the fcc-bcc-fluid triple point and the phase boundaries in its vicinity are in good agreement with the prediction supplied by a recently proposed corresponding-states principle [J. Chem. Phys. 134, 241101 (2011); Europhys. Lett. 100, 66004 (2012)]. The particularly strong resemblance between the behavior of the UFM and GCM models are also discussed.96317CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESPCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESSem informação2013/08293-7Sem informaçãoWe gratefully acknowledge support from the Brazilian agencies CNPq, Fapesp, Capes, and the Center for Computational Engineering and Sciences-Fapesp/Cepid Grant No. 2013/08293-7. Part of the calculations were performed at CCJDR-IFGW-UNICAMP and CENAPAD-SP. The authors acknowledge the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil) for providing HPC resources of the SDumont supercomputer, which have contributed to the research results reported in this paper. URL: http://sdumont.lncc.b
The irreducible unitary representations of the extended Poincare group in (1+1) dimensions
We prove that the extended Poincare group in (1+1) dimensions is
non-nilpotent solvable exponential, and therefore that it belongs to type I. We
determine its first and second cohomology groups in order to work out a
classification of the two-dimensional relativistic elementary systems.
Moreover, all irreducible unitary representations of the extended Poincare
group are constructed by the orbit method. The most physically interesting
class of irreducible representations corresponds to the anomaly-free
relativistic particle in (1+1) dimensions, which cannot be fully quantized.
However, we show that the corresponding coadjoint orbit of the extended
Poincare group determines a covariant maximal polynomial quantization by
unbounded operators, which is enough to ensure that the associated quantum
dynamical problem can be consistently solved, thus providing a physical
interpretation for this particular class of representations.Comment: 12 pages, Revtex 4, letter paper; Revised version of paper published
in J. Math. Phys. 45, 1156 (2004
Multi-drug and β-lactam resistance in Escherichia coli and food-borne pathogens
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