5,333 research outputs found
Reappraising the Spite Lithium Plateau: Extremely Thin and Marginally Consistent with WMAP
The lithium abundance in 62 halo dwarfs is determined from accurate
equivalent widths reported in the literature and an improved infrared flux
method (IRFM) temperature scale. The Li abundance of 41 plateau stars (those
with Teff > 6000 K) is found to be independent of temperature and metallicity,
with a star-to-star scatter of only 0.06 dex over a broad range of temperatures
(6000 K < Teff < 6800 K) and metallicities (-3.4 < [Fe/H] < -1), thus imposing
stringent constraints on depletion by mixing and production by Galactic
chemical evolution. We find a mean Li plateau abundance of A(Li) = 2.37 dex
(7Li/H = 2.34 X 10^{-10}), which, considering errors of the order of 0.1 dex in
the absolute abundance scale, is just in borderline agreement with the
constraints imposed by the theory of primordial nucleosynthesis and WMAP data
(2.51 < A(Li)[WMAP] < 2.66 dex).Comment: ApJ Letters, in pres
Kinetic energy of protons in ice Ih and water: a path integral study
The kinetic energy of H and O nuclei has been studied by path integral
molecular dynamics simulations of ice Ih and water at ambient pressure. The
simulations were performed by using the q-TIP4P/F model, a point charge
empirical potential that includes molecular flexibility and anharmonicity in
the OH stretch of the water molecule. Ice Ih was studied in a temperature range
between 210-290 K, and water between 230-320 K. Simulations of an isolated
water molecule were performed in the range 210-320 K to estimate the
contribution of the intramolecular vibrational modes to the kinetic energy. Our
results for the proton kinetic energy, K_H, in water and ice Ih show both
agreement and discrepancies with different published data based on deep
inelastic neutron scattering experiments. Agreement is found for water at the
experimental melting point and in the range 290-300 K. Discrepancies arise
because data derived from the scattering experiments predict in water two
maxima of K_H around 270 K and 277 K, and that K_H is lower in ice than in
water at 269 K. As a check of the validity of the employed water potential, we
show that our simulations are consistent with other experimental thermodynamic
properties related to K_H, as the temperature dependence of the liquid density,
the heat capacity of water and ice at constant pressure, and the isotopic shift
in the melting temperature of ice upon isotopic substitution of either H or O
atoms. Moreover, the temperature dependence of K_H predicted by the q-TIP4P/F
model for ice Ih is found to be in good agreement to results of path integral
simulations using ab initio density functional theory.Comment: 11 pages, 6 figures, 2 table
Isotope effects on the lattice parameter of cubic SiC
Path-integral molecular dynamics simulations in the isothermal-isobaric (NPT)
ensemble have been carried out to study the dependence of the lattice parameter
of 3C-SiC upon isotope mass. This computational method allows a quantitative
and nonperturbative study of such anharmonic effect. Atomic nuclei were treated
as quantum particles interacting via a tight-binding-type potential. At 300 K,
the difference Delta a between lattice parameters of 3C-SiC crystals with 12C
and 13C amounts to 2.1 x 10^{-4} A. The effect due to Si isotopes is smaller,
and amounts to 3.5 x 10^{-5} A when replacing 28Si by 29Si. Results of the PIMD
simulations are interpreted in terms of a quasiharmonic approximation for the
lattice vibrations.Comment: 4 pages, 3 figure
Efecto de la Corriente y Longitud de Arco en Soldaduras con Arco Eléctrico Asistido por Modelado Matemático
A 2D mathematical model was developed for the GTAW arc welding process (Gas Tungsten Arc Welding). Computational simulations were performed by using the commercial software PHOENICS based on mass and momentum conservation equations as well as on Maxwell equations. The model predicts the electric characteristics of the arc column, flow patterns, temperature profiles, heat flux, total heat flow and the electrical potential, by varying the arc length and the applied current. By increasing the current the arc jet is stronger, hotter and provides more heat to the weld pool, while by increasing the arc length the maximum temperature, maximum velocity and heat flow are unchanged, although a short arc focuses the heat in a small area and a long arc spreads the heat in a wider area of the work piece.Keywords: Electric arc, heat transfer, fluid flow, mathematical modeling.
Study of the lepton flavor-violating decay
The lepton flavor violating decay is studied in the
context of several extended models that predict the existence of the new gauge
boson named . A calculation of the strength of the lepton flavor
violating coupling is presented by using the most general
renormalizable Lagrangian that includes lepton flavor violation. We used the
experimental value of the muon magnetic dipole moment to bound this coupling,
from which the parameter
is constrained and it is found that
for a
boson mass of 2 TeV. Alongside, we employed the experimental
restrictions over the and processes
in the context of several models that predict the existence of the
gauge boson to bound the mentioned coupling. The most restrictive bounds come
from the calculation of the three-body decay. For this case, it was found that
the most restrictive result is provided by a vector-like coupling, denoted as
, for the case, finding around for a
boson mass of 2 TeV. We used this information to estimate the
branching ratio for the decay. According to the analyzed
models the least optimistic result is provided by the Sequential model,
which is of the order of for a boson mass around 2 TeV.Comment: Revised versio
Asymptotic solvers for ordinary differential equations with multiple frequencies
We construct asymptotic expansions for ordinary differential equations with highly oscillatory forcing terms, focusing on the case of multiple, non-commensurate frequencies. We derive an asymptotic expansion in inverse powers of the oscillatory parameter and use its truncation as an exceedingly effective means to discretize the differential equation in question. Numerical examples illustrate the effectiveness of the method
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