16,127 research outputs found
alpha :a constant that is not a constant?
We review the observational information on the constancy of the fine
structure constant alpha. We find that small improvements on the measurement of
^{187}Re lifetime can provide significant progress in exploring the range of
variability suggested by QSO data. We also discuss the effects of a time
varying alpha on stellar structure and evolution. We find that radioactive
dating of ancient stars can offer a new observational window.Comment: 9 pages with 3 ps figures included, to appear on the Proc. of
ESO-CERN-ESA Symposium on Astronomy, Cosmology and Fundamental Physics,
Garching bei Munchen, Germany March 4-7, 200
Linear Solar Models
We present a new approach to study the properties of the sun. We consider
small variations of the physical and chemical properties of the sun with
respect to Standard Solar Model predictions and we linearize the structure
equations to relate them to the properties of the solar plasma. By assuming
that the (variation of) the present solar composition can be estimated from the
(variation of) the nuclear reaction rates and elemental diffusion efficiency in
the present sun, we obtain a linear system of ordinary differential equations
which can be used to calculate the response of the sun to an arbitrary
modification of the input parameters (opacity, cross sections, etc.). This new
approach is intended to be a complement to the traditional methods for solar
model calculation and allows to investigate in a more efficient and transparent
way the role of parameters and assumptions in solar model construction. We
verify that these Linear Solar Models recover the predictions of the
traditional solar models with an high level of accuracy.Comment: 29 pages, 10 figure
Properties of (GaIn)O over the whole range
Using density-functional ab initio theoretical techniques, we study
(GaIn)O in both its equilibrium structures (monoclinic
and bixbyite) and over the whole range of composition. We establish
that the alloy exhibits a large and temperature-independent miscibility gap. On
the low- side, the favored phase is isostructural with -GaO;
on the high- side, it is isostructural with bixbyite InO. The
miscibility gap opens between approximately 15\% and 55\% In content for the
bixbyite alloy grown epitaxially on InO, and 15\% and 85\% In content
for the free-standing bixbyite alloy. The gap, volume and band offsets to the
parent compound also exhibit anomalies as function of . Specifically, the
offsets in epitaxial conditions are predominantly type-B staggered, but have
opposite signs in the two end-of-range phases.Comment: 7 pages, 4 figure
Helioseismic tests of diffusion theory
We present a quantitative estimate of the accuracy of the calculated
diffusion coefficients, by comparing predictions of solar models with
observational data provided by helioseismology. By taking into account the
major uncertainties in building solar models we conclude that helioseismology
confirms the diffusion efficiency adopted in SSM calculations, to the 10%
level.Comment: 5 pages with 1 ps figure included, LaTeX file with l-aa.sty,
submitted to Astronomy and Astrophysic
Interactions of the solar neutrinos with the deuterons
Starting from chiral Lagrangians, possessing the SU(2)_L x SU(2)_R local
chiral symmetry, we derive weak axial one-boson exchange currents in the
leading order in the 1/M expansion (M is the nucleon mass). We apply these
currents in calculations of the cross sections for the disintegration of the
deuterons by the low energy neutrinos. The nuclear wave functions are derived
from a variant of the OBEPQB potential and from the Nijmegen 93 and Nijmegen I
nucleon-nucleon interactions. The comparison of our cross sections with those
obtained within the pionless effective field theory and other potential model
calculations shows that the solar neutrino-deuteron cross sections can be
calculated within an accuracy of 3.3 %.Comment: 6 pages, 1 figure, 6 tables, conference tal
Structure and gap of low- (GaIn)O alloys
We study the electronic and local structural properties of pure and
In-substituted -GaO using density functional theory (DFT). Our
main result is that the structural energetics of In in GaO causes most
sites to be essentially inaccessible to In substitution, thus reducing the
maximum In content in thi to somewhere between 12 and 25 \% in this phase. We
also find that the gap variation with doping is essentially due to "chemical
pressure", i.e. volume variations with doping.Comment: 4 pages, 3 figure
A gyrokinetic model for the plasma periphery of tokamak devices
A gyrokinetic model is presented that can properly describe strong flows,
large and small amplitude electromagnetic fluctuations occurring on scale
lengths ranging from the electron Larmor radius to the equilibrium
perpendicular pressure gradient scale length, and large deviations from thermal
equilibrium. The formulation of the gyrokinetic model is based on a second
order description of the single charged particle dynamics, derived from Lie
perturbation theory, where the fast particle gyromotion is decoupled from the
slow drifts, assuming that the ratio of the ion sound Larmor radius to the
perpendicular equilibrium pressure scale length is small. The collective
behavior of the plasma is obtained by a gyrokinetic Boltzmann equation that
describes the evolution of the gyroaveraged distribution function and includes
a non-linear gyrokinetic Dougherty collision operator. The gyrokinetic model is
then developed into a set of coupled fluid equations referred to as the
gyrokinetic moment hierarchy. To obtain this hierarchy, the gyroaveraged
distribution function is expanded onto a velocity-space Hermite-Laguerre
polynomial basis and the gyrokinetic equation is projected onto the same basis,
obtaining the spatial and temporal evolution of the Hermite-Laguerre expansion
coefficients. The Hermite-Laguerre projection is performed accurately at
arbitrary perpendicular wavenumber values. Finally, the self-consistent
evolution of the electromagnetic fields is described by a set of gyrokinetic
Maxwell's equations derived from a variational principle, with the velocity
integrals of the gyroaveraged distribution function explicitly evaluated
Helioseismology and Beryllium neutrino
We derive a lower limit on the Beryllium neutrino flux on earth,
, in the absence of oscillations,
by using helioseismic data, the B-neutrino flux measured by Superkamiokande and
the hydrogen abundance at the solar center predicted by Standard Solar Model
(SSM) calculations. We emphasize that this abundance is the only result of SSMs
needed for getting . We also derive lower bounds for the
Gallium signal, SNU, and for the Chlorine signal,
SNU, which are about above their
corresponding experimental values, SNU and SNU.Comment: 10 pages plus 1 ps figure, RevTeX styl
- …