The Subpulse Modulation Properties of Pulsars and its Frequency Dependence

A large sample of about two hundred pulsars have been observed to study their subpulse modulation at an observing wavelength of (when achievable) both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting subpulses are discovered for the first time and are confirmed for many others. This leads to the conclusion that it could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. Drifting subpulses are in general found at both frequencies and the measured values of P3 at the two frequencies are highly correlated, showing the broadband nature of this phenomenon. Also the modulation indices measured at the two frequencies are clearly correlated, although at 92 cm they are on average possibly higher. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission is composed out of a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. There is no obvious correlation found between P3 and the pulsar age (or any other pulsar parameter) contrary to reports in the past.Comment: Proceedings of the 40 Years of Pulsars: Millisecond Pulsars, Magnetars and More conference in Montrea

The Stability of Double White Dwarf Binaries Undergoing Direct Impact Accretion

We present numerical simulations of dynamically unstable mass transfer in a double white dwarf binary with initial mass ratio, q = 0.4. The binary components are approximated as polytropes of index n = 3/2 and the initially synchronously rotating, semi-detached equilibrium binary is evolved hydrodynamically with the gravitational potential being computed through the solution of Poisson's equation. Upon initiating deep contact in our baseline simulation, the mass transfer rate grows by more than an order of magnitude over approximately ten orbits, as would be expected for dynamically unstable mass transfer. However, the mass transfer rate then reaches a peak value, the binary expands and the mass transfer event subsides. The binary must therefore have crossed the critical mass ratio for stability against dynamical mass transfer. Despite the initial loss of orbital angular momentum into the spin of the accreting star, we find that the accretor's spin saturates and angular momentum is returned to the orbit more efficiently than has been previously suspected for binaries in the direct impact accretion mode. To explore this surprising result, we directly measure the critical mass ratio for stability by imposing artificial angular momentum loss at various rates to drive the binary to an equilibrium mass transfer rate. For one of these driven evolutions, we attain equilibrium mass transfer and deduce that effectively q_crit has evolved to approximately 2/3. Despite the absence of a fully developed disk, tidal interactions appear effective in returning excess spin angular momentum to the orbit.Comment: 27 pages, 6 figures. Please see http://www.phys.lsu.edu/faculty/tohline/astroph/mftd07/ for animations and full resolution figures. Accepted for publication in the Astrophysical Journa

EXPLORING MANAGEMENT OF TEACHING AND ASSESSMENT IN MIXED CLASS OF LEARNERS: CASE OF SELECTED SECONDARY SCHOOLS IN VHUMBEDZI CIRCUIT

This study investigated the management of teaching and assessment of the “qualified to be progressed” learners (QP) for quality learning and teaching. Mixed methods approach was adopted. Random and purposive sampling were used to select teachers and School Management Team (SMT) member respectively who formed the study sample. The study was conducted in 10 purposively selected secondary schools in the Vhumbedzi Circuit, Limpopo, South Africa. Data was collected from the 165 selected teachers through the use of questionnaires while interviews were conducted for 10 SMTs. The quantitative data was analysed using percentage, while the qualitative data was coded and thematically analysed. The finding of the study showed that there are no clearly developed strategies to help manage the teaching and assessment of the QP learners. The study recommends that the DBE, through its curriculum section, should assist the SMTs in the development of strategies to help manage the teaching and assessment of the QP learners for quality teaching and learning

Theory of Underdoped Cuprates

We develop a slave-boson theory for the t-J model at finite doping which respects an SU(2) symmetry -- a symmetry previously known to be important at half filling. The mean field phase diagram is found to be consistent with the phases observed in the cuprate superconductors, which contains d-wave superconductor, spin gap, strange metal, and Fermi liquid phases. The spin gap phase is best understood as the staggered flux phase, which is nevertheless translationally invariant for physical quantities. The electron spectral function shows small Fermi pockets at low doping which continuously evolve into the large Fermi surface at high doping concentrations.Comment: 4 pages, latex(revtex,epsf), 3 figure

Gestion intégrée de l'eau au sein d'un bassin versant

Le développement de méthodes et d'outils pour l'aide à la gestion et la planification des ressources en eau est un projet fondateur de l'unité DIVHA. L'environnement de modélisation actuel, HyD2002, voulu très générique, permet de représenter différents types d'aménagements et de simuler divers modes d'allocation en présence d'usages et de règles multiples. Nous présenterons le principe de conception à partir d'un exemple d'aménagements à buts multiples dans le sud de la France. Nous détaillerons les divers modes de gestion pouvant être simulé. Le modèle de base d'HyD2002 permet de simuler l'application de consignes de gestion sur de longues chroniques d'apports, considérées comme représentatives de l'aléa hydrologique. L'évaluation se fait a posteriori par une analyse intensité-durée-fréquence des écarts par rapport aux objectifs. Les consignes de gestion sont ainsi testées par de nombreuses simulations. Afin de réduire ces tâtonnements, il apparaît séduisant d'avoir recours à l'optimisation numérique. Mais les limitations sont nombreuses et la simulation reste essentielle pour évaluer les gains réels dans une gestion opérationnelle. Différentes applications de l'outil sont présentées : aide à la planification des eaux en Guadeloupe, réhabilitation de l'alimentation en eau du château de Versailles, tests de diverses allocations des ressources dans le bassin de l'Indus au Pakistan et dans le bassin du Walawe au Sri Lanka. Différentes évolutions de l'outil, au niveau des modèles d'allocation, et au niveau de la nécessaire collaboration avec des modèles d'offre et de demande, sont envisagées afin de faciliter l'analyse de l'équité et de la viabilité d'alternatives de gestion. (Résumé d'auteur