Current sheets at three-dimensional magnetic nulls:effect of compressibility

The nature of current sheet formation in the vicinity of three-dimensional (3D) magnetic null points is investigated. The particular focus is upon the effect of the compressibility of the plasma on the qualitative and quantitative properties of the current sheet. An initially potential 3D null is subjected to shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas, in press (2007)]. It is found that as the incompressible limit is approached, the collapse of the null point is suppressed, and an approximately planar current sheet aligned to the fan plane is present instead. This is the case regardless of whether the spine or fan of the null is sheared. Both the peak current and peak reconnection rate are reduced. The results have a bearing on previous analytical solutions for steady-state reconnection in incompressible plasmas, implying that fan current sheet solutions are dynamically accessible, while spine current sheet solutions are not.Comment: to appear in Physics of Plasmas. This version contains updated figures and references, additional discussion, and typos are fixed. This is the second in a series of papers - the first of which (by the same authors) is located at astro-ph/0701462. A version with higher quality figures can be found at http://www.maths.dundee.ac.uk/~dpontin

Surface Effects on the Mechanical Elongation of AuCu Nanowires: De-alloying and the Formation of Mixed Suspended Atomic Chains

We report here an atomistic study of the mechanical deformation of AuxCu(1-x) atomic-size wires (NWs) by means of high resolution transmission electron microscopy (HRTEM) experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.Comment: Accepted to Journal of Applied Physics (JAP

The GRB 030328 host: another case of a blue starburst galaxy

We present for the first time the detection of the GRB 030328 host galaxy in four optical bands equivalent to UBRI. The host galaxy spectral energy distribution is consistent with a low extinction (E(B-V) < 0.21) starburst galaxy. The restframe B-band magnitude of the host is M_B ~ -20.4Comment: 4 pages, 2 figures, accepted for publication in Il nuovo cimento (4th Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004

Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts

We report high resolution transmission electron microscopy and ab initio calculation results for the defect formation in Pt nanocontacts (NCs). Our results show that there is a size limit to the existence of twins (extended structural defects). Defects are always present but blocked away from the tip axes. The twins may act as scattering plane, influencing contact electron transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure

A young stellar environment for the superluminous supernova PTF12dam

The progenitors of super luminous supernovae (SLSNe) are still a mystery. Hydrogen-poor SLSN hosts are often highly star-forming dwarf galaxies and the majority belongs to the class of extreme emission line galaxies hosting young and highly star-forming stellar populations. Here we present a resolved long-slit study of the host of the hydrogen-poor SLSN PTF12dam probing the kpc environment of the SN site to determine the age of the progenitor. The galaxy is a "tadpole" with uniform properties and the SN occurred in a star-forming region in the head of the tadpole. The galaxy experienced a recent star-burst superimposed on an underlying old stellar population. We measure a very young stellar population at the SN site with an age of ~3 Myr and a metallicity of 12+log(O/H)=8.0 at the SN site but do not observe any WR features. The progenitor of PTF12dam must have been a massive star of at least 60 M_solar and one of the first stars exploding as a SN in this extremely young starburst.Comment: submitted to MNRAS letters. 5 pages, 3 figures, supplementary material: 2 figures, 2 table

Eficacia del programa de intervención funcional del lenguaje Flip-N en niños con TEL

105 p.Este trabajo de investigación estudia la eficacia del programa de intervención funcional FLIP-N en el desempeño narrativo de un grupo de 6 niños cuyas edades fluctúan entre 5.4 y 6 años de edad, todos con diagnóstico de trastorno específico del lenguaje (TEL). Los seis sujetos se distribuyeron en dos grupos, 3 en el grupo control (sin intervención) y los otros 3 en el grupo experimental (con intervención), con pre y post-test. La intervención consistió en un total de 14 sesiones, una sesión a la semana con una duración aproximada de 40 a 60 minutos cada una de ellas. Se realizaron cuatro evaluaciones inter-sesiones aplicadas a ambos grupos. Se realizó un análisis descriptivo de los resultados. Los sujetos pertenecientes al grupo experimental lograron mejorar su desempeño narrativo, aunque este no fue en la magnitud esperada. Los sujetos del grupo control presentaron un comportamiento fluctuante, en donde uno de ellos resultó descendido en relación a su evaluación pre-test y los otros dos sujetos evidenciaron avances en su rendimiento narrativo. Se discuten las implicancias clínicas y los posibles factores que influirían en los resultados obtenidos. Palabras clave: trastorno específico del lenguaje, intervención, discurso narrativo, Programa FLIP-N./ABSTRACT: The aim of this work was to study the effect of the intervention program FLIP-N on the narrative performance of a group of six children aged between 5.5 and 6.0 years, all of them with a diagnosis of Specific Language Impairment (SLI). The six subjects were divided in two groups. Three of them were included in the control group (without intervention), and the other three were included in the experimental group (with intervention). A pretest-intervention-postest design was used. The intervention phase included 14 sessions, one session per week lasting from 40 to 60 minutes each. Four assessments were applied to both groups during the intervention phase. A descriptive analysis of the results was performed. The subjects in the experimental group improved their narrative performance, but it did not reach the expected magnitude. The control group showed a fluctuating behavior, because one of the children was dropped in relation to the pretest, but the other two subjects showed progress in their narrative performance. Clinical implications, and the possible factors involved are discussed. Key terms: Specific Language Impairment, intervention, narrative discourse, FLIP-N Program