Jet Collimation by Small-Scale Magnetic Fields

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the ``hoop stress'' of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic ``hoop stress'' is balanced either by the gas pressure of the jet, or by the centrifugal force if the jet is spinning. Since the length-scale of the magnetic field is small (< the cross-sectional radius of the jet << the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.Comment: 14 pages, no figur

Building analytical three-field cosmological models

A difficult task to deal with is the analytical treatment of models composed by three real scalar fields, once their equations of motion are in general coupled and hard to be integrated. In order to overcome this problem we introduce a methodology to construct three-field models based on the so-called "extension method". The fundamental idea of the procedure is to combine three one-field systems in a non-trivial way, to construct an effective three scalar field model. An interesting scenario where the method can be implemented is within inflationary models, where the Einstein-Hilbert Lagrangian is coupled with the scalar field Lagrangian. We exemplify how a new model constructed from our method can lead to non-trivial behaviors for cosmological parameters.Comment: 11 pages, and 3 figures, updated version published in EPJ

First order formalism for thick branes in modified gravity with Lagrange multiplier

This work discuss the construction of braneworld solutions in modified gravity with Lagrange multipliers. We examine the general aspects of the model and present a first order formalism that help us to find analytic solutions of the equations of motion. We also investigate some explicit models, analyse linear stability of the metric and comment on how to relate models investigated in other works to the ones examined in the present study.Comment: 7 pages, 9 figures, it matches the published versio

Are Magnetic Wind-Driving Disks Inherently Unstable?

There have been claims in the literature that accretion disks in which a centrifugally driven wind is the dominant mode of angular momentum transport are inherently unstable. This issue is considered here by applying an equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated, magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves for this class of models typically exhibit two distinct branches. It is argued that only one of these branches represents unstable equilibria and that a real disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20). Uses emulateapj.cl

Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height

The formation of mounded surfaces in epitaxial growth is attributed to the presence of barriers against interlayer diffusion in the terrace edges, known as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth using a ES barrier explicitly dependent on the step height. Our model has an intrinsic topological step barrier even in the absence of an explicit ES barrier. We show that mounded morphologies can be obtained even for a small barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma equation, is observed in absence of an explicit step barrier. The mounded surfaces are described by a super-roughness dynamical scaling characterized by locally smooth (faceted) surfaces and a global roughness exponent $\alpha>1$. The thin film limit is featured by surfaces with self-assembled three-dimensional structures having an aspect ratio (height/width) that may increase or decrease with temperature depending on the strength of step barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary materia

How daily negative affect and emotional exhaustion correlates with work engagement and presenteeism-constrained productivity

Exploring productivity loss due to presenteeism and how it can have an influence on employees’ productivity has been an important concern for human resource managers. Using the conservation of resources theory and the job demands-resources model, this study investigated how work engagement could mediate the relationship between both emotional exhaustion and negative affect and productivity loss due to presenteeism in a country experiencing an economic downturn. A total of 42 employees from a health care institution completed a 10-day diary survey (420 diary tasks). Multilevel linear modeling results, including power analyses, showed that negative affect and emotional exhaustion predicted productivity loss due to presenteeism positively, whereas work engagement predicted productivity loss negatively. Furthermore, we found that at a day-level, work engagement mediated the effects of emotional exhaustion and negative affect on productivity loss due to presenteeism. This study highlights the importance of promoting work engagement to reduce the effects of negative affect and emotional exhaustion on productivity loss due to presenteeism through intervention.info:eu-repo/semantics/acceptedVersio

Lorentz-breaking effects in scalar-tensor theories of gravity

In this work, we study the effects of breaking Lorentz symmetry in scalar-tensor theories of gravity taking torsion into account. We show that a space-time with torsion interacting with a Maxwell field by means of a Chern-Simons-like term is able to explain the optical activity in syncrotron radiation emitted by cosmological distant radio sources. Without specifying the source of the dilaton-gravity, we study the dilaton-solution. We analyse the physical implications of this result in the Jordan-Fierz frame. We also analyse the effects of the Lorentz breaking in the cosmic string formation process. We obtain the solution corresponding to a cosmic string in the presence of torsion by keeping track of the effects of the Chern-Simons coupling and calculate the charge induced on this cosmic string in this framework. We also show that the resulting charged cosmic string gives us important effects concerning the background radiation.The optical activity in this case is also worked out and discussed.Comment: 10 pages, no figures, ReVTex forma

Afilamento e forma do tronco de árvores de Eucalyptus grandis e Eucalyptus saligna e suas variações com a adubação.

Este trabalho analisa possíveis alterações no afilamento médio e fator de forma de árvores de Eucalyptus saligna e Eucalyptus grandis pelo efeito de aplicação de fertilizantes no momento de plantio e parcelada durante a rotação. Os experimentos de adubação, com estas espécies foram implantados na região dos cerrados, nos municípios de Brotas e Altinópolis, Estado de São Paulo. Os tratamentos foram selecionados devido à elevada resposta à adubação obtida ao final da rotação. Os resultados obtidos para afilamento médio e fator de forma, de parcelas e árvores adubadas e não adubadas, possibilitaram as seguintes conclusões: a) O afilamento médio das parcelas diminuiu do quarto para o sexto ano e posteriormente aumentou para o oitavo ano. Aparentemente, estas variações deveram-se a uma redução na taxa de crescimento em altura e um aumento na taxa de crescimento em diâmetro, durante o mesmo período. b) A adubação melhorou levemente a forma das árvores do Eucalyptus grandis, e este efeito está indiretamente relacionado com as diferentes quantidades de fertilizantes devido ao seu efeito no crescimento em altura das árvores. c) As variações no afilamento médio das árvores podem ser atribuídas às alterações nos padrões de crescimento em altura e diâmetro. Variáveis diferentes que alteram a conicidade das árvores, devem ser primeiramente analisadas em relação à sua influência no crescimento em altura e diâmetro, antes de serem consideradas como fatores isolados. d) Nenhuma alteração foi verificada, na relação entre o diâmetro à altura do peito e na metade da altura da árvore, que pudesse ser atribuída às doses ou métodos de aplicação de fertilizantes. e) Não foi possível obter-se uma regressão precisa para estimar fatores de forma baseada nas variáveis estudadas neste trabalho

Alteracoes limnologicas no Rio Paraguai ("dequada") e o fenomeno natural de mortandade de peixes no Pantanal Mato-Grossense - MS.

No Pantanal, ocorre um fenomeno natural de deterioracao da qualidade da agua, denominado regionalmente como "Dequada", relacionado a decomposicao da grande massa de materia organica submersa no inicio do processo de inundacao. Sua magnitude e dependente das caracteristicas do pulso de inundacao, ou seja, caracteristicas da fase de seca anterior e do periodo de inundacao subsequente (volume e velocidade). De acordo com essa magnitude, pode provocar mortandade massiva de peixes (podendo alcancar a ordem de milhares de toneladas), docorrente da deplecao de oxigenio e do aumento da concentracao de gas carbonico, resultantes dos processos de oxidacao da materia organica, tanto nos campos inundados, quanto na coluna d'agua dos rios. Os peixes moribundos apresentam comportamento tipico de estresse respiratorio. Outros fatores, como gas sulfidrico, amonia, aluminio, sodio e potassio (solidos dissolvidos), alteracoes de pH e compostos provenientes da atividade fitoplanctonica foram descardados, pois nao foram encontrados em niveis considerados toxicos para peixes. Pesticidas e matais pesados nao foram analisados, mas seus niveis dificilmente poderiam ser responsaveis por mortandades tao grandes em uma area tao extensa, onde a atividade antropica ainda e reduzida. Em 1995, o fenomeno foi muito expressivo, devido a rapidez e ao grande volume da onda de cheia, chegando a provocar a total anoxia e/ou manter sub hipoxia o rio Paraguai por mais dois meses e apresentar valores de gas carbonico dissociado de ate 79mg/L, nunca antes obtidos.bitstream/item/37450/1/BP07.pd