2,556 research outputs found
Structure of hard-hypersphere fluids in odd dimensions
The structural properties of single component fluids of hard hyperspheres in
odd space dimensionalities are studied with an analytical approximation
method that generalizes the Rational Function Approximation earlier introduced
in the study of hard-sphere fluids [S. B. Yuste and A. Santos, Phys. Rev. A
{\bf 43}, 5418 (1991)]. The theory makes use of the exact form of the radial
distribution function to first order in density and extends it to finite
density by assuming a rational form for a function defined in Laplace space,
the coefficients being determined by simple physical requirements. Fourier
transform in terms of reverse Bessel polynomials constitute the mathematical
framework of this approximation, from which an analytical expression for the
static structure factor is obtained. In its most elementary form, the method
recovers the solution of the Percus-Yevick closure to the Ornstein-Zernike
equation for hyperspheres at odd dimension. The present formalism allows one to
go beyond by yielding solutions with thermodynamic consistency between the
virial and compressibility routes to any desired equation of state. Excellent
agreement with available computer simulation data at and is
obtained. As a byproduct of this study, an exact and explicit polynomial
expression for the intersection volume of two identical hyperspheres in
arbitrary odd dimensions is given.Comment: 18 pages, 7 figures; v2: new references added plus minor changes; to
be published in PR
Probing the Structure of Halo Nuclei
Our understanding of halo nuclei has so far relied on high-energy scattering
and reactions, but a number of uncertainties remain. I discuss in general terms
the new range of observables which will be measured by experiments around the
Coulomb barrier, and how some details of the reaction mechanisms still need to
be clarified.Comment: Proceedings of FUSION97 conference (March 1997), South Durras,
Australia. Submitted to J. Physics G: special issue `Heavy ion collisions at
near barrier energies'. No figures; uses IOPConf.sty (included
CNO behaviour in planet-harbouring stars. II. Carbon abundances in stars with and without planets using the CH band
Context. Carbon, oxygen and nitrogen (CNO) are key elements in stellar
formation and evolution, and their abundances should also have a significant
impact on planetary formation and evolution.
Aims. We present a detailed spectroscopic analysis of 1110 solar-type stars,
143 of which are known to have planetary companions. We have determined the
carbon abundances of these stars and investigate a possible connection between
C and the presence of planetary companions. Methods. We used the HARPS
spectrograph to obtain high-resolution optical spectra of our targets. Spectral
synthesis of the CH band at 4300\AA was performed with the spectral synthesis
codes MOOG and FITTING.
Results. We have studied carbon in several reliable spectral windows and have
obtained abundances and distributions that show that planet host stars are
carbon rich when compared to single stars, a signature caused by the known
metal-rich nature of stars with planets. We find no different behaviour when
separating the stars by the mass of the planetary companion.
Conclusions. We conclude that reliable carbon abundances can be derived for
solar-type stars from the CH band at 4300\AA. We confirm two different slope
trends for [C/Fe] with [Fe/H] because the behaviour is opposite for stars above
and below solar values. We observe a flat distribution of the [C/Fe] ratio for
all planetary masses, a finding that apparently excludes any clear connection
between the [C/Fe] abundance ratio and planetary mass.Comment: 10 pages, 10 figures. Accepted to A&
C/O vs Mg/Si ratios in solar type stars: The HARPS sample
Aims. We present a detailed study of the Mg/Si and C/O ratios and their
importance in determining the mineralogy of planetary companions. Methods.
Using 499 solar-like stars from the HARPS sample, we determine C/O and Mg/Si
elemental abundance ratios to study the nature of the possible planets formed.
We separated the planetary population in low-mass planets ( < 30 ) and high-mass planets ( > 30 ) to test for possible
relation with the mass. Results. We find a diversity of mineralogical ratios
that reveal the different kinds of planetary systems that can be formed, most
of them dissimilar to our solar system. The different values of the Mg/Si and
C/O ratios can determine different composition of planets formed. We found that
100\% of our planetary sample present C/O < 0.8. 86\% of stars with high-mass
companions present 0.8 > C/O > 0.4, while 14\% present C/O values lower than
0.4. Regarding Mg/Si, all stars with low-mass planetary companion showed values
between 1 and 2, while 85% of the high-mass companion sample does. The other
15\% showed Mg/Si values below 1. No stars with planets were found with Mg/Si >
2. Planet hosts with low-mass companions present C/O and Mg/Si ratios similar
to those found in the Sun, whereas stars with high-mass companions have lower
C/O.Comment: 9 pages, 12 figues. Accepted in A&
Long range absorption in the scattering of 6He on 208Pb and 197Au at 27 MeV
Quasi-elastic scattering of 6He at E_lab=27 MeV from 197Au has been measured
in the angular range of 6-72 degrees in the laboratory system employing LEDA
and LAMP detection systems. These data, along with previously analysed data of
6He + 208Pb at the same energy, are analyzed using Optical Model calculations.
The role of Coulomb dipole polarizability has been investigated. Large
imaginary diffuseness parameters are required to fit the data. This result is
an evidence for long range absorption mechanisms in 6He induced reactions.Comment: 10 pages, 10 figures, minor corrections. To appear in Nucl. Phys.
Inherent Rheology of a Granular Fluid in Uniform Shear Flow
In contrast to normal fluids, a granular fluid under shear supports a steady
state with uniform temperature and density since the collisional cooling can
compensate locally for viscous heating. It is shown that the hydrodynamic
description of this steady state is inherently non-Newtonian. As a consequence,
the Newtonian shear viscosity cannot be determined from experiments or
simulation of uniform shear flow. For a given degree of inelasticity, the
complete nonlinear dependence of the shear viscosity on the shear rate requires
the analysis of the unsteady hydrodynamic behavior. The relationship to the
Chapman-Enskog method to derive hydrodynamics is clarified using an approximate
Grad's solution of the Boltzmann kinetic equationComment: 10 pages, 4 figures; substantially enlarged version; to be published
in PR
Class of dilute granular Couette flows with uniform heat flux
In a recent paper [F. Vega Reyes et al., Phys. Rev. Lett. 104, 028001 (2010)]
we presented a preliminary description of a special class of steady Couette
flows in dilute granular gases. In all flows of this class the viscous heating
is exactly balanced by inelastic cooling. This yields a uniform heat flux and a
linear relationship between the local temperature and flow velocity. The class
(referred to as the LTu class) includes the Fourier flow of ordinary gases and
the simple shear flow of granular gases as special cases. In the present paper
we provide further support for this class of Couette flows by following four
different routes, two of them being theoretical (Grad's moment method of the
Boltzmann equation and exact solution of a kinetic model) and the other two
being computational (molecular dynamics and Monte Carlo simulations of the
Boltzmann equation). Comparison between theory and simulations shows a very
good agreement for the non-Newtonian rheological properties, even for quite
strong inelasticity, and a good agreement for the heat flux coefficients in the
case of Grad's method, the agreement being only qualitative in the case of the
kinetic model.Comment: 15 pages, 10 figures; v2: change of title plus some other minor
change
Hombre entre los hombres
El siguiente trabajo podría considerarse como un capítulo de filosofía pedagógica y titularse "cómo enseñar al joven a hacerse hombre", o también "hombre entre los hombres". En su sencillez quisiera ser una respuesta al tema que me ha sido señalado: "Metodología de las Ciencias Sociológicas". Las verdades de la Sociología son verdades para ser entendidas y para ser vividas; y sólo cuando son vividas puede afirmarse que han llegado de veras a ser entendida
Hombre entre los hombres
El siguiente trabajo podría considerarse como un capítulo de filosofía pedagógica y titularse "cómo enseñar al joven a hacerse hombre", o también "hombre entre los hombres". En su sencillez quisiera ser una respuesta al tema que me ha sido señalado: "Metodología de las Ciencias Sociológicas". Las verdades de la Sociología son verdades para ser entendidas y para ser vividas; y sólo cuando son vividas puede afirmarse que han llegado de veras a ser entendida
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