71 research outputs found
The Propagation of Magneto-Centrifugally Launched Jets: I
We present simulations of the propagation of magnetized jets. This work
differs from previous studies in that the cross-sectional distributions of the
jets's state variables are derived from analytical models for
magneto-centrifugal launching. The source is a magnetized rotator whose
properties are specfied as boundary conditions. The jets in these simulations
are considerably more complex than the ``top-hat''constant density etc.
profiles used in previous work. We find that density and magnetic field
stratification (with radius) in the jet leads to new behavior including the
separation of an inner jet core from a low density collar. We find this {\it
jet within a jet} structure, along with the magnetic stresses, leads to
propagation behaviors not observed in previous simulation studies. Our
methodology allows us to compare MHD jets from different types of sources whose
properties could ultimately be derived from the behavior of the propagating
jets.Comment: 42 pages, accepted by the Ap
Protostellar Evolution during Time Dependent, Anisotropic Collapse
The formation and collapse of a protostar involves the simultaneous infall
and outflow of material in the presence of magnetic fields, self-gravity, and
rotation. We use self-similar techniques to self-consistently model the
anisotropic collapse and outflow by a set of angle-separated self-similar
equations. The outflow is quite strong in our model, with the velocity
increasing in proportion to radius, and material formally escaping to infinity
in the finite time required for the central singularity to develop.
Analytically tractable collapse models have been limited mainly to
spherically symmetric collapse, with neither magnetic field nor rotation. Other
analyses usually employ extensive numerical simulations, or either perturbative
or quasistatic techniques. Our model is unique as an exact solution to the
non-stationary equations of self-gravitating MHD, which features co-existing
regions of infall and outflow.
The velocity and magnetic topology of our model is quadrupolar, although
dipolar solutions may also exist. We provide a qualitative model for the origin
and subsequent evolution of such a state. However, a central singularity forms
at late times, and we expect the late time behaviour to be dominated by the
singularity rather than to depend on the details of its initial state. Our
solution may, therefore, have the character of an attractor among a much more
general class of self-similarity.Comment: 11 pages, 5 figures, To appear in MNRAS, Memorial paper for M.
Aburiha
Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets
The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap
Etude d'un nouveau virus à ARN isolé de Turnaca rufisquamata (Lépidoptère Notodontidae) défoliateur du palmier à huile, en Côte-d'Ivoire
Les larves d'une nouvelle espèce d'insecte défoliateur, Turnaca rufisquamata (Lépidoptère Notodontidae), sont apparues en pullulation sur la plantation de palmiers à huile Aghien à La Mé (Côte-d'Ivoire). Le déclenchement d'une épizootie nous a permis de mettre en évidence un entomovirus particulièrement pathogène. Ce virus se caractérise par sa forme icosaédrique et son diamètre de 30 nm. Le spectre photométrique d'une suspension virale purifiée montre un rapport des densités optiques à 260 et 280 nm de 1,83. L'étude électrophorétique de sa composition polypeptidique fait apparaître trois protéines majeures, PV1, PV2 et PV3, et une mineure, PV4, dont les poids moléculaires sont respectivement de 41000, 33000, 30000 et 12000. Le génome viral est constitué d'un fragment d'ARN dont le poids moléculaire est estimé à 9,4 Kb. Les propriétés physico-chimiques de ce virus l'apparentent à la famille des Picornaviridae. A partir d'anticorps produits sur lapin, des tests d'immunodiffusion montrent que ce virus est distinct des autres Picornavirus d'insecte
3D Relativistic Magnetohydrodynamic Simulations of Magnetized Spine-Sheath Relativistic Jets
Numerical simulations of weakly magnetized and strongly magnetized
relativistic jets embedded in a weakly magnetized and strongly magnetized
stationary or weakly relativistic (v = c/2) sheath have been performed. A
magnetic field parallel to the flow is used in these simulations performed by
the new GRMHD numerical code RAISHIN used in its RMHD configuration. In the
numerical simulations the Lorentz factor jet is precessed to
break the initial equilibrium configuration. In the simulations sound speeds
are in the weakly magnetized simulations and in the strongly magnetized simulations. The Alfven wave speed is
in the weakly magnetized simulations and in
the strongly magnetized simulations. The results of the numerical simulations
are compared to theoretical predictions from a normal mode analysis of the
linearized relativistic magnetohydrodynamic (RMHD) equations capable of
describing a uniform axially magnetized cylindrical relativistic jet embedded
in a uniform axially magnetized relativistically moving sheath. The theoretical
dispersion relation allows investigation of effects associated with maximum
possible sound speeds, Alfven wave speeds near light speed and relativistic
sheath speeds. The prediction of increased stability of the weakly magnetized
system resulting from c/2 sheath speeds and the stabilization of the strongly
magnetized system resulting from c/2 sheath speeds is verified by the numerical
simulation results.Comment: 31 pages, 8 figures, accepted for publicatin in ApJ. A paper with
high resolution figures available at
http://gammaray.nsstc.nasa.gov/~mizuno/research_new.htm
A Global Jet/Circulation Model for Young Stars
Powerful, highly collimated jets, surrounded by bipolar molecular outflows,
are commonly observed near Young Stellar Objects (YSOs). In the usual
theoretical picture of star formation, a jet is ejected from a magnetized
accretion disk, with a molecular outflow being driven either by the jet or by a
wider wind coming from the disk. Here, we propose an alternative global model
for the flows surrounding YSOs. In addition to a central accretion-ejection
engine driving the jet, the molecular outflow is powered by the infalling
matter and follows a circulation pattern around the central object without
necessarily being entrained by a jet. It is shown that the model produces a
heated pressure-driven outflow with magneto-centrifugal acceleration and
collimation. We report solutions for the three different parts of this
self-similar model, i.e. the jet, the infalling envelope and the circulating
matter that eventually forms the molecular outflow. This new picture of the
accretion/outflow phase provides a possible explanation for several observed
properties of YSO outflows. The most relevant ones are the presence of high
mass molecular outflows around massive protostars, and a realistic fraction
(typically 0.1) of the accretion flow that goes into the jet.Comment: accepted for publication in Astronomy and Astrophysic
A disk-wind model with correct crossing of all MHD critical surfaces
The classical Blandford & Payne (1982) model for the magnetocentrifugal
acceleration and collimation of a disk-wind is revisited and refined. In the
original model, the gas is cold and the solution is everywhere subfast
magnetosonic. In the present model the plasma has a finite temperature and the
self-consistent solution of the MHD equations starts with a subslow
magnetosonic speed which subsequently crosses all critical points, at the slow
magnetosonic, Alfven and fast magnetosonic separatrix surfaces. The superfast
magnetosonic solution thus satisfies MHD causality. Downstream of the fast
magnetosonic critical point the poloidal streamlines overfocus towards the axis
and the solution is terminated. The validity of the model to disk winds
associated with young stellar objects is briefly discussed. ~Comment: 13 pages, MNRAS accepted for publicatio
Dissipationless Disk Accretion
We consider disk accretion resulting purely from the loss of angular momentum
due to the outflow of plasma from a magnetized disk. In this limiting case, the
dissipation due to the viscosity and finite electrical conductivity of the
plasma can be neglected. We have obtained self-consistent, self-similar
solutions for dissipationless disk accretion. Such accretion may result in the
formation of objects whose bolometric luminosities are lower than the flux of
kinetic energy in the ejected material.Comment: 17 pages, 6 figures, published in Astronomy Reports, Vol.49, No.1,
2005, p.57 (submitted September 13, 2003). Unfortunately, we did not upload
the paper to astro-ph before, but since the topic is now of interest we feel
that the paper would benefit the communit
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