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 (Ga1−x_{1-x}Inx_x)2_2O3_3 over the whole xx range

Using density-functional ab initio theoretical techniques, we study (Ga$_{1-x}$In$_x$)$_2$O$_3$ in both its equilibrium structures (monoclinic $\beta$ 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-$x$ side, the favored phase is isostructural with $\beta$-Ga$_2$O$_3$; on the high-$x$ side, it is isostructural with bixbyite In$_2$O$_3$. The miscibility gap opens between approximately 15\% and 55\% In content for the bixbyite alloy grown epitaxially on In$_2$O$_3$, 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 $x$. 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-xx (Ga1−x_{1-x}Inx_x)2_2O3_3 alloys

We study the electronic and local structural properties of pure and In-substituted $\beta$-Ga$_2$O$_3$ using density functional theory (DFT). Our main result is that the structural energetics of In in Ga$_2$O$_3$ 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, $\Phi(Be)_{min} = 1\cdot 10^9 cm^{-2} s^{-1}$, 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 $\Phi(Be)_{min}$. We also derive lower bounds for the Gallium signal, $G_{min}=(91 \pm 3)$ SNU, and for the Chlorine signal, $C_{min}=(3.24\pm 0.14)$ SNU, which are about $3\sigma$ above their corresponding experimental values, $G_{exp}= (72\pm 6)$ SNU and $C_{exp}= (2.56\pm 0.22)$ SNU.Comment: 10 pages plus 1 ps figure, RevTeX styl