Dust cleansing of star-forming gas: II. Did late accretion flows change the chemical composition of the solar atmosphere?

The possibility that the chemical composition of the solar atmosphere has been affected by radiative dust cleansing of late and weak accretion flows by the proto-sun itself, is explored. Estimates, using semi-analytical methods and numerical simulations of the motion of dust grains in a collapsing non-magnetic and non-rotating gas sphere with a central light source are made, to model possible dust-cleansing effects. Our calculations indicate that the amounts of cleansed material may well be consistent with the abundance differences observed for the Sun when compared with solar-like stars and with the relations found between these differences and condensation temperature of the element. It seems quite possible that the proposed mechanism might have produced the significant abundance effects observed for the Sun, provided that late and relatively weak accretion did occur. The effects of cleansing may, however, be affected by outflows from the Sun, the existence and dynamics of magnetic fields and of the accretion disk, and the possible presence and location of the Early Sun in a rich stellar cluster.Comment: 10 pages, accepted for publication in Astronomy & Astrophysics, version with corrected typos and added information on magnetic field

Nucleon electromagnetic structure: past, present, and future

We present the experimental status of electromagnetic hadron form factors. New and surprising results, based on polarization measurements, have been recently obtained for the electric proton and neutron form factors. In particular, the electric and magnetic distributions inside the proton appear not to be the same, in disagreement with results extracted from the unpolarized cross section, using the Rosenbluth separation. The new findings have given rise to a large number of papers and different speculations, as they question directly the models of nucleon structure and the reaction mechanism itself (based on $1\gamma$-exchange), with a possible revision of the calculation of radiative corrections, two-photon contribution etc. New data in time-like region are also available, through annihilation reactions. A large interest in this field arises, due also to the possibility of new measurements in polarized electron nucleon elastic scattering at JLab, and also in the time-like region, at Frascati and at the future FAIR international facility.Comment: 10 pages, 3 figures, to be included in a special issue of Nuovo Cimento

On the intrinsic limitation of the Rosenbluth method at large momentum transfer

Correlations in the elastic electron proton scattering data show that the Rosenbluth method is not reliable for the extraction of the electric proton form factors at large momentum transfer, where the magnetic term dominates, due to the size and the $\epsilon$ dependence of the radiative corrections.Comment: 10 pages, 4 figure

Physical bounds and radiation modes for MIMO antennas

Modern antenna design for communication systems revolves around two extremes: devices, where only a small region is dedicated to antenna design, and base stations, where design space is not shared with other components. Both imply different restrictions on what performance is realizable. In this paper properties of both ends of the spectrum in terms of MIMO performance is investigated. For electrically small antennas the size restriction dominates the performance parameters. The regions dedicated to antenna design induce currents on the rest of the device. Here a method for studying fundamental bound on spectral efficiency of such configurations is presented. This bound is also studied for $N$-degree MIMO systems. For electrically large structures the number of degrees of freedom available per unit area is investigated for different shapes. Both of these are achieved by formulating a convex optimization problem for maximum spectral efficiency in the current density on the antenna. A computationally efficient solution for this problem is formulated and investigated in relation to constraining parameters, such as size and efficiency

Using the hierarchical linear model to understand school production in South Africa

The emphasis placed in the existing South African school production function literature on better skilled teachers and better school management is discussed. Ordinary least squares and hierarchical linear production function models, using 2000 SACMEQ data, for the country and for a sub-set of historically disadvantaged schools, are constructed. Ways of making the results more readable for policymakers are explored. The importance of physical infrastructure, textbook and nutrition budgets is highlighted by the models. Correct allocation of teaching and management time in schools, less learner repetition, and better teaching methodologies stand out as important school and classroom management imperatives.Educational quality, Education policy, Education resources, SACMEQ, South Africa