16,161 research outputs found
Kinematic Constraints to the Transition Redshift from SNe Ia Union Data
The kinematic approach to cosmological tests provides a direct evidence to
the present accelerating stage of the universe which does not depend on the
validity of general relativity, as well as on the matter-energy content of the
Universe. In this context, we consider here a linear two-parameter expansion
for the decelerating parameter, , where and are
arbitrary constants to be constrained by the Union supernovae data. By assuming
a flat Universe we find that the best fit to the pair of free parameters is
() = ( whereas the transition redshift is () (). This
kinematic result is in agreement with some independent analyzes and
accommodates more easily many dynamical flat models (like CDM).Comment: 10 pages, 4 figures, 1 tabl
Counterrotation in magnetocentrifugally driven jets and other winds
Rotation measurement in jets from T Tauri stars is a rather difficult task.
Some jets seem to be rotating in a direction opposite to that of the underlying
disk, although it is not yet clear if this affects the totality or part of the
outflows. On the other hand, Ulysses data also suggest that the solar wind may
rotate in two opposite ways between the northern and southern hemispheres. We
show that this result is not as surprising as it may seem and that it emerges
naturally from the ideal MHD equations. Specifically, counterrotating jets
neither contradict the magnetocentrifugal driving of the flow nor prevent
extraction of angular momentum from the disk. The demonstration of this result
is shown by combining the ideal MHD equations for steady axisymmetric flows.
Provided that the jet is decelerated below some given threshold beyond the
Alfven surface, the flow will change its direction of rotation locally or
globally. Counterrotation is also possible for only some layers of the outflow
at specific altitudes along the jet axis. We conclude that the counterrotation
of winds or jets with respect to the source, star or disk, is not in
contradiction with the magnetocentrifugal driving paradigm. This phenomenon may
affect part of the outflow, either in one hemisphere, or only in some of the
outflow layers. From a time-dependent simulation, we illustrate this effect and
show that it may not be permanent.Comment: To appear in ApJ
Counter-rotation in relativistic magnetohydrodynamic jets
Young stellar object observations suggest that some jets rotate in the
opposite direction with respect to their disk. In a recent study, Sauty et al.
(2012) have shown that this does not contradict the magnetocentrifugal
mechanism that is believed to launch such outflows. Signatures of motions
transverse to the jet axis and in opposite directions have recently been
measured in M87 (Meyer et al. 2013). One possible interpretation of this motion
is the one of counter rotating knots. Here, we extend our previous analytical
derivation of counter-rotation to relativistic jets, demonstrating that
counter-rotation can indeed take place under rather general conditions. We show
that both the magnetic field and a non-negligible enthalpy are necessary at the
origin of counter-rotating outflows, and that the effect is associated with a
transfer of energy flux from the matter to the electromagnetic field. This can
be realized in three cases : if a decreasing enthalpy causes an increase of the
Poynting flux, if the flow decelerates, or, if strong gradients of the magnetic
field are present. An illustration of the involved mechanism is given by an
example of relativistic MHD jet simulation.Comment: Accepted for publication in ApJ
Physical approximations for the nonlinear evolution of perturbations in dark energy scenarios
The abundance and distribution of collapsed objects such as galaxy clusters
will become an important tool to investigate the nature of dark energy and dark
matter. Number counts of very massive objects are sensitive not only to the
equation of state of dark energy, which parametrizes the smooth component of
its pressure, but also to the sound speed of dark energy as well, which
determines the amount of pressure in inhomogeneous and collapsed structures.
Since the evolution of these structures must be followed well into the
nonlinear regime, and a fully relativistic framework for this regime does not
exist yet, we compare two approximate schemes: the widely used spherical
collapse model, and the pseudo-Newtonian approach. We show that both
approximation schemes convey identical equations for the density contrast, when
the pressure perturbation of dark energy is parametrized in terms of an
effective sound speed. We also make a comparison of these approximate
approaches to general relativity in the linearized regime, which lends some
support to the approximations.Comment: 15 pages, 2 figure
Solarização, biofumigação e uso de rizobactérias no controle de Meloidogyne incognita em pimenta.
Brane World Dynamics and Adiabatic Matter creation
We have treated the adiabatic matter creation process in various three-brane
models by applying thermodynamics of open systems. The matter creation rate is
found to affect the evolution of scale factor and energy density of the
universe. We find modification at early stages of cosmic dynamics. In GB and RS
brane worlds, by chosing appropriate parameters we obtain standard scenario,
while the warped DGP model has different Friedmann equations. During later
stages, since the matter creation is negligible the evolution reduces to FRW
expansion, in RS and GB models.Comment: Replaced with TEX file,No figures, Corrected References and typos.
Accepted in IJMP
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