22,675 research outputs found
Global Alfven Wave Heating of the Magnetosphere of Young Stars
Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to
explain plasma heating in the magnetosphere of young stars. The wave and basic
plasma parameters are compatible with the requirement that the dissipation
length of GAWs be comparable to the distance between the shocked region at the
star's surface and the truncation region in the accretion disk. A two-fluid
magnetohydrodynamic plasma model is used in the analysis. A current carrying
filament along magnetic field lines acts as a waveguide for the GAW. The
current in the filament is driven by plasma waves along the magnetic field
lines and/or by plasma crossing magnetic field lines in the truncated region of
the disk of the accreting plasma. The conversion of a small fraction of the
kinetic energy into GAW energy is sufficient to heat the plasma filament to
observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure
The role of damped Alfven waves on magnetospheric accretion models of young stars
We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa
Density functional investigations of defect induced mid-gap states in graphane
We have carried out ab initio electronic structure calculations on graphane
(hydrogenated graphene) with single and double vacancy defects. Our analysis of
the density of states reveal that such vacancies induce the mid gap states and
modify the band gap. The induced states are due to the unpaired electrons on
carbon atoms. Interestingly the placement and the number of such states is
found to be sensitive to the distance between the vacancies. Furthermore we
also found that in most of the cases the vacancies induce a local magnetic
moment.Comment: 15 page
Some addition formulae for Abelian functions for elliptic and hyperelliptic curves of cyclotomic type
We discuss a family of multi-term addition formulae for Weierstrass functions
on specialized curves of genus one and two with many automorphisms. In the
genus one case we find new addition formulae for the equianharmonic and
lemniscate cases, and in genus two we find some new addition formulae for a
number of curves, including the Burnside curve.Comment: 19 pages. We have extended the Introduction, corrected some typos and
tidied up some proofs, and inserted extra material on genus 3 curve
The Possibility of Thermal Instability in Early-Type Stars Due to Alfven Waves
It was shown by dos Santos et al. the importance of Alfv\'en waves to explain
the winds of Wolf-Rayet stars. We investigate here the possible importance of
Alfv\'en waves in the creation of inhomogeneities in the winds of early-type
stars. The observed infrared emission (at the base of the wind) of early-type
stars is often larger than expected. The clumping explains this characteristic
in the wind, increasing the mean density and hence the emission measure, making
possible to understand the observed infrared, as well as the observed
enhancement in the blue wing of the line. In this study, we
investigate the formation of these clumps a via thermal instability. The
heat-loss function used, , includes physical processes such as:
emission of (continuous and line) recombination radiation; resonance line
emission excited by electron collisions; thermal bremsstrahlung; Compton
heating and cooling; and damping of Alfv\'en waves. As a result of this
heat-loss function we show the existence of two stable equilibrium regions. The
stable equilibrium region at high temperature is the diffuse medium and at low
temperature the clumps. Using this reasonable heat-loss function, we show that
the two stable equilibrium regions can coexist over a narrow range of pressures
describing the diffuse medium and the clumps.Comment: 21 pages (psfig.sty), 5 figures (included), ApJ accepted. Also
available at http://www.iagusp.usp.br/preprints/preprint.htm
A influência da presença de ferro endógeno na remediação de diesel via reagente de Fenton modificado.
Este trabalho avaliou a eficiência catalítica do ferro endógeno na reação de Fenton para a remediação de diferentes solos tropicais contaminados com o óleo diesel com diferentes teores das seguintes formas de ferro endógenos: amorfo, pedogenético, biodisponível e total. Em 70% de degradação do óleo diesel. Íons férricos encontrados, naturalmente, nos solos brasileiros também catalisam a decomposição do H2O2 para geração de radicais hidroxilas. Contudo, foi observado que a taxa de remediação é dependente das formas de ferro encontradas nos solos
Alfvenic Heating of Protostellar Accretion Disks
We investigate the effects of heating generated by damping of Alfven waves on
protostellar accretion disks. Two mechanisms of damping are investigated,
nonlinear and turbulent, which were previously studied in stellar winds
(Jatenco-Pereira & Opher 1989a, b). For the nominal values studied, f=delta
v/v_{A}=0.002 and F=varpi/Omega_{i}=0.1, where delta v, v_{A} and varpi are the
amplitude, velocity and average frequency of the Alfven wave, respectively, and
Omega_{i} is the ion cyclotron frequency, we find that viscous heating is more
important than Alfven heating for small radii. When the radius is greater than
0.5 AU, Alfvenic heating is more important than viscous heating. Thus, even for
the relatively small value of f=0.002, Alfvenic heating can be an important
source of energy for ionizing protostellar disks, enabling angular momentum
transport to occur by the Balbus-Hawley instability.Comment: 21 pages, 9 figures. Accepted for publication in Ap
Longevidade pós-colheita de hastes florais de híbridos de abacaxi ornamental tratadas com soluções de condicionamento.
O abacaxi (A. comosus (L.) Merrill) é o representante de maior importância econômica da família bromeliácea e se constitui na sexta fruteira tropical mais explorada economicamente no mundo (FAO, 2010) como alimento. Entretanto seu uso ornamental vem crescendo nos últimos anos, constituindo-se em um produto de alto potencial para o segmento da floricultura, já sendo cultivado no Ceará e Rio Grande do Norte com vistas à exportação (Brainer & Oliveira, 2007). Como a demanda por novidades neste setor é alta, a Embrapa Mandioca e Fruticultura tem um programa de melhoramento genético para a geração de novos híbridos desde 2003 e com materiais já em etapa de validação agronômica (Souza et al., 2006; 2009)
Cancellation of atmospheric turbulence effects in entangled two-photon beams
Turbulent airflow in the atmosphere and the resulting random fluctuations in
its refractive index have long been known as a major cause of image
deterioration in astronomical imaging and figures among the obstacles for
reliable optical communication when information is encoded in the spatial
profile of a laser beam. Here we show that using correlation imaging and a
suitably prepared source of photon pairs, the most severe of the disturbances
inflicted on the beam by turbulence can be cancelled out. Other than a
two-photon light source, only linear passive optical elements are needed and,
as opposed to adaptive optics techniques, our scheme does not rely on active
wavefront correction.Comment: 5 pages, 3 figure
Uso de cera à base de carnaúba em hastes de abacaxizeiros ornamentais (Ananas comosus var. erectifolius).
A demanda pelo abacaxi ornamental é crescente nos últimos anos. O processo de lavagem das hastes, que retira a cera natural, é exigência do mercado externo, já que proporciona um produto de maior qualidade e aceitação. Este trabalho objetivou determinar a melhor diluição e forma de aplicação de cera à base de carnaúba em hastes florais de abacaxi ornamental, visando aumento da sua longevidade
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