4,452 research outputs found
On the Influence of Magnetic Fields on the Structure of Protostellar Jets
We here present the first results of fully three-dimensional (3-D) MHD
simulations of radiative cooling pulsed (time-variable) jets for a set of
parameters which are suitable for protostellar outflows. Considering different
initial magnetic field topologies in approximate with the
thermal gas, i.e., (i) a longitudinal, and (ii) a helical field, both of which
permeating the jet and the ambient medium; and (iii) a purely toroidal field
permeating only the jet, we find that the overall morphology of the pulsed jet
is not very much affected by the presence of the different magnetic field
geometries in comparison to a nonmagnetic calculation. Instead, the magnetic
fields tend to affect essentially the detailed structure and emission
properties behind the shocks at the head and at the pulse-induced internal
knots, particularly for the helical and toroidal geometries. In these cases, we
find, for example, that the emissivity behind the internal knots can
be about three to four times larger than that of the purely hydrodynamical jet.
We also find that some features, like the nose cones that often develop at the
jet head in 2-D calculations involving toroidal magnetic fields, are smoothed
out or absent in the 3-D calculations.Comment: 13 pages, 3 figures, Accepted by ApJ Letters after minor corrections
(for high resolution figures, see http://www.iagusp.usp.br/~adriano/h.tar
Magnetic Field Effects on the Head Structure of Protostellar Jets
We present the results of 3-D SPMHD numerical simulations of
supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic
configurations are considered: (i) a helical and (ii) a longitudinal field. We
find that magnetic fields have important effects on the dynamics and structure
of radiative cooling jets, especially at the head. The presence of a helical
field suppresses the formation of the clumpy structure which is found to
develop at the head of purely hydrodynamical jets. On the other hand, a cooling
jet embedded in a longitudinal magnetic field retains clumpy morphology at its
head. This fragmented structure resembles the knotty pattern commonly observed
in HH objects behind the bow shocks of HH jets. This suggests that a strong
(equipartition) helical magnetic field configuration is ruled out at the jet
head. Therefore, if strong magnetic fields are present, they are probably
predominantly longitudinal in those regions. In both magnetic configurations,
we find that the confining pressure of the cocoon is able to excite
short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks
along the beam. These shocks are not strong however, and it likely that they
could only play a secondary role in the formation of the bright knots observed
in HH jets.Comment: 14 pages, 2 Gif figures, uses aasms4.sty. Also available on the web
page http://www.iagusp.usp.br/preprints/preprint.html. To appear in The
Astrophysical Journal Letter
HIV Infection Diagnosis - What Changed in 10 Years?
Introdução: A infecção VIH/SIDA mantém-se uma importante causa de morbilidade e mortalidade nos países em vias de desenvolvimento, onde a terapêutica antiretroviral só está disponível para um número restrito de doentes. Nos países desenvolvidos, há tendência para estabilização da epidemia mas é ainda frequente o diagnóstico em fases tardias, com subaproveitamento dos benefícios do tratamento, aqui facilmente acessível. Em Portugal, país da Europa Ocidental com maior prevalência de infecção VIH, continua a verificar-se o aumento de novos casos.
Objectivos e Métodos: Avaliação retrospectiva com caracterização
e comparação de dois grupos de doentes com infecção VIH acompanhados em consulta de um hospital central de Lisboa - um com diagnóstico em 1997/1998, outro com diagnóstico em 2007/2008 – com o objectivo de analisar possíveis diferenças de “perfil” demográfico, epidemiológico, clínico-laboratorial e terapêutico. Resultados: Foram estudados 74 doentes com diagnóstico em 1997/1998 e 106 doentes com diagnóstico em 2007/2008. A idade média foi superior em 2007/2008. O sexo masculino predominou em qualquer dos períodos, sem diferença significativa. A raça negra e a origem não portuguesa tiveram maior representatividade em 2007/2008. Verificou-se um aumento da transmissão heterossexual e um menor número de casos associados ao
uso de drogas endovenosas em 2007/2008. O estadio inicial não mostrou diferenças estatisticamente significativas nos dois
períodos. Em 1997/1998 os esquemas terapêuticos envolveram maior número de comprimidos e maior número de tomas diárias.
Conclusões: Parece-nos fulcral o investimento em estratégias de redução de riscos e de diagnóstico precoce, em cuja definição devem ser contemplados múltiplos factores, individuais, comportamentais
e sociais. Com uma prevenção mais efectiva e início atempado da terapêutica, que se objectiva progressivamente mais potente, mais simples e mais bem tolerada, e se aspira
de acesso universal, talvez um dia se alcance o tão desejável controlo da epidemia
Modeling the spectrum of gravitational waves in the primordial Universe
Recent observations from type Ia Supernovae and from cosmic microwave
background (CMB) anisotropies have revealed that most of the matter of the
Universe interacts in a repulsive manner, composing the so-called dark energy
constituent of the Universe. The analysis of cosmic gravitational waves (GW)
represents, besides the CMB temperature and polarization anisotropies, an
additional approach in the determination of parameters that may constrain the
dark energy models and their consistence. In recent work, a generalized
Chaplygin gas model was considered in a flat universe and the corresponding
spectrum of gravitational waves was obtained. The present work adds a massless
gas component to that model and the new spectrum is compared to the previous
one. The Chaplygin gas is also used to simulate a -CDM model by means
of a particular combination of parameters so that the Chaplygin gas and the
-CDM models can be easily distinguished in the theoretical scenarios
here established. The lack of direct observational data is partialy solved when
the signature of the GW on the CMB spectra is determined.Comment: Proc. of the Conference on Magnetic Fields in the Universe: from
laboratories and stars to primordial structures, AIP(NY), eds. E. M. de
Gouveia Dal Pino, G. Lugones & A. Lazarian (2005), in press. (8 pages, 11
figures
On the existence of 0/1 polytopes with high semidefinite extension complexity
In Rothvo\ss{} it was shown that there exists a 0/1 polytope (a polytope
whose vertices are in \{0,1\}^{n}) such that any higher-dimensional polytope
projecting to it must have 2^{\Omega(n)} facets, i.e., its linear extension
complexity is exponential. The question whether there exists a 0/1 polytope
with high PSD extension complexity was left open. We answer this question in
the affirmative by showing that there is a 0/1 polytope such that any
spectrahedron projecting to it must be the intersection of a semidefinite cone
of dimension~2^{\Omega(n)} and an affine space. Our proof relies on a new
technique to rescale semidefinite factorizations
Severe Invasive Streptococcal Disease - Emerging Disease?
Introdução: A incidência da doença estreptocócica invasiva (DSI) tem vindo a aumentar na Europa e América do Norte
desde o final dos anos 1980, provavelmente relacionada com a emergência de estirpes mais virulentas. Em oito meses foram
internados no nosso hospital seis casos desta entidade rara.
Objectivos: Descrever as características da doença estreptocócica invasiva grave.
Métodos: Estudo descritivo, de Dezembro de 2007 a Julho de 2008. Analisaram-se parâmetros demográficos, factores de
risco, clínica, terapêutica, complicações e evolução.
Resultados: Identificaram-se seis casos com mediana de idade de 2,5 anos: síndrome de choque tóxico estreptocócico (STSS)
(2), fasceíte necrotizante (2), bacteriémia (1) e infecção estreptocócica
grave (1). Cinco casos ocorreram entre Dezembro e Fevereiro. Em quatro doentes registaram-se eventuais factores de risco (infecções virais e anti-inflamatórios não esteroides). Duas crianças necessitaram de cirurgia e três de tratamento em
cuidados intensivos. Todas as estirpes eram susceptíveis à penicilina
e clindamicina. Ocorreram complicações em 5/6 doentes: choque séptico (3), coagulação intravascular disseminada (2),
insuficiência renal (2), abcesso de tecidos moles (2), sobreinfecção
bacteriana (2), síndrome de dificuldade respiratória do adulto (1), osteomielite /artrite séptica (1) e pneumonia/derrame
pleural (1). Não se registaram óbitos.
Comentários: Seis casos de DSI num curto espaço de tempo podem indiciar a emergência de estirpes de Streptococcus
grupo A (GAS) de maior virulência no nosso país, pelo que a realização de estudos moleculares será fundamental na identificação de clones invasivos. Apesar da susceptibilidade à penicilina, a DSI cursa com morbilidade elevada, pelo que, o
importante parece ser procurar novas formas de tratar o doente e não o agente
The precession of the giant HH34 outflow: a possible jet deceleration mechanism
The giant jets represent a fundamental trace of the historical evolution of
the outflow activity over timescales which are comparable to the accretion time
of the outflow sources in their main protostellar phase. The study of such huge
jets provides the possibility of retrieving important elements related to the
life of the outflow sources. In this paper, we study the role of precession
(combined with jet velocity-variability and the resulting enhanced interaction
with the surrounding environment) as a deceleration mechanism for giant jets
using a numerical approach. We obtain predictions of H alpha intensity maps and
position-velocity diagrams from 3D simulations of the giant HH 34 jet
(including an appropriate ejection velocity time-variability and a precession
of the outflow axis), and we compare them with previously published
observations of this object. Our simulations represent a step forward from
previous numerical studies of HH objects, in that the use of a 7-level, binary
adaptive grid has allowed us to compute models which appropiately cover all
relevant scales of a giant jet, from the ~ 100 AU jet radius close to the
source to the ~ 1 pc length of the outflow. A good qualitative and quantitative
agreement is found between the model predictions and the observations.
Moreover, we show that a critical parameter for obtaining a better or worse
agreement with the observations is the ratio rho_j/rho_a between the jet and
the environmental densities. The implications of this result in the context of
the current star formation models are discussed (ABRIDGED).Comment: 19 pages, 8 eps figs.,uses aaspp4; accepted by the Ap
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