527 research outputs found
Jet simulations extending radially self-similar MHD models
We perform a numerical simulation of magnetohydrodynamic radially
self-similar jets, whose prototype is the Blandford & Payne analytical example.
The reached final steady state is valid close to the rotation axis and also at
large distances above the disk where the classical analytical model fails to
provide physically acceptable solutions. The outflow starts with a sub-slow
magnetosonic speed which subsequently crosses all relevant MHD critical points
and corresponding magnetosonic separatrix surfaces. The characteristics are
plotted together with the Mach cones and the super-fast magnetosonic outflow
satisfies MHD causality. The final solution remains close enough to the
analytical one which is thus shown to be topologically stable and robust for
various boundary conditions.Comment: 11 pages, 8 figures, minor changes to match the version accepted by
MNRA
Synthetic synchrotron emission maps from MHD models for the jet of M87
We present self-consistent global, steady-state MHD models and synthetic
optically thin synchrotron emission maps for the jet of M87. The model consist
of two distinct zones: an inner relativistic outflow, which we identify with
the observed jet, and an outer cold disk-wind. While the former does not
self-collimate efficiently due to its high effective inertia, the latter
fulfills all the conditions for efficient collimation by the
magneto-centrifugal mechanism. Given the right balance between the effective
inertia of the inner flow and the collimation efficiency of the outer disk
wind, the relativistic flow is magnetically confined into a well collimated
beam and matches the measurements of the opening angle of M87 over several
orders of magnitude in spatial extent. The synthetic synchrotron maps reproduce
the morphological structure of the jet of M87, i.e. center-bright profiles near
the core and limb-bright profiles away from the core. At the same time, they
also show a local increase of brightness at some distance along the axis
associated to a recollimation shock in the MHD model. Its location coincides
with the position of the optical knot HST-1. In addition our best fitting model
is consistent with a number of observational constraints such as the magnetic
field in the knot HST-1, and the jet-to-counterjet brightness ratio.Comment: 9 pages, 9 figures, accepted by Ap
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
Stability and structure of analytical MHD jet formation models with a finite outer disk radius
(Abridged) Finite radius accretion disks are a strong candidate for launching
astrophysical jets from their inner parts and disk-winds are considered as the
basic component of such magnetically collimated outflows. The only available
analytical MHD solutions for describing disk-driven jets are those
characterized by the symmetry of radial self-similarity. Radially self-similar
MHD models, in general, have two geometrical shortcomings, a singularity at the
jet axis and the non-existence of an intrinsic radial scale, i.e. the jets
formally extend to radial infinity. Hence, numerical simulations are necessary
to extend the analytical solutions towards the axis and impose a physical
boundary at finite radial distance. We focus here on studying the effects of
imposing an outer radius of the underlying accreting disk (and thus also of the
outflow) on the topology, structure and variability of a radially self-similar
analytical MHD solution. The initial condition consists of a hybrid of an
unchanged and a scaled-down analytical solution, one for the jet and the other
for its environment. In all studied cases, we find at the end steady
two-component solutions.Comment: 14 pages, 15 figures, accepted for publication in A &
Magnetic collimation of the relativistic jet in M87
We apply a two-zone MHD model to the jet of M87. The model consists of an
inner relativistic outflow, which is surrounded by a non-relativistic outer
disk-wind. The outer disk-wind collimates very well through magnetic
self-collimation and confines the inner relativistic jet into a narrow region
around the rotation axis. Further, we show by example, that such models
reproduce very accurately the observed opening angle of the M87 jet over a
large range from the kiloparsec scale down to the sub-parsec scale.Comment: 4 pages, 2 figures, accepted by A&A Letter
Systematic Construction of Exact 2-D MHD Equilibria with Steady, Compressible Flow in Cartesian Geometry and Uniform Gravity
We present a systematic method for constructing two-dimensional
magnetohydrodynamic equilibria with compressible flow in Cartesian geometry.
This systematic method has already been developed in spherical geometry and
applied in modelling solar and stellar winds and outflows (Vlahakis &
Tsinganos,1998) but is derived here in Cartesian geometry in the context of the
solar atmosphere for the first time. Using the method we find several new
classes of solutions, some of which generalise known solutions, including the
Kippenhahn & Schl\"uter (1957) and Hood & Anzer (1990) solar prominence models
and the Tsinganos, Surlantzis & Priest (1993) coronal loop model with flow, and
some of which are completely new. Having developed the method in full and
summarised the several classes of solutions, we explore in a some detail one of
the classes to illustrate the general construction method. From one of the new
classes of solutions we calculate two loop-like solutions, one of which is the
first exact two-dimensional magnetohydrodynamic equilibrium with
trans-Alfv\'enic flow.Comment: 12 pages, 12 Postscript figures, accepted by A&A 16th November 2001,
uses aa.cl
Resistive jet simulations extending radially self-similar magnetohydrodynamic models
Numerical simulations with self-similar initial and boundary conditions
provide a link between theoretical and numerical investigations of jet
dynamics. We perform axisymmetric resistive magnetohydrodynamic (MHD)
simulations for a generalised solution of the Blandford & Payne type, and
compare them with the corresponding analytical and numerical ideal-MHD
solutions. We disentangle the effects of the numerical and physical
diffusivity. The latter could occur in outflows above an accretion disk, being
transferred from the underlying disk into the disk corona by MHD turbulence
(anomalous turbulent diffusivity), or as a result of ambipolar diffusion in
partially ionized flows. We conclude that while the classical magnetic Reynolds
number measures the importance of resistive effects in the
induction equation, a new introduced number, \rbeta=(\beta/2)R_{\rm m} with
the plasma beta, measures the importance of the resistive effects in
the energy equation. Thus, in magnetised jets with , when \rbeta \la
1 resistive effects are non-negligible and affect mostly the energy equation.
The presented simulations indeed show that for a range of magnetic
diffusivities corresponding to \rbeta \ga 1 the flow remains close to the
ideal-MHD self-similar solution.Comment: Accepted for publication in MNRA
Resistive MHD jet simulations with large resistivity
Axisymmetric resistive MHD simulations for radially self-similar initial
conditions are performed, using the NIRVANA code. The magnetic diffusivity
could occur in outflows above an accretion disk, being transferred from the
underlying disk into the disk corona by MHD turbulence (anomalous turbulent
diffusivity), or as a result of ambipolar diffusion in partially ionized flows.
We introduce, in addition to the classical magnetic Reynolds number Rm, which
measures the importance of resistive effects in the induction equation, a new
number Rb, which measures the importance of the resistive effects in the energy
equation. We find two distinct regimes of solutions in our simulations. One is
the low-resistivity regime, in which results do not differ much from ideal-MHD
solutions. In the high-resistivity regime, results seem to show some
periodicity in time-evolution, and depart significantly from the ideal-MHD
case. Whether this departure is caused by numerical or physical reasons is of
considerable interest for numerical simulations and theory of astrophysical
outflows and is currently investigated.Comment: To appear in the proceedings of the "Protostellar Jets in Context"
conference held on the island of Rhodes, Greece (7-12 July 2008
Recommended from our members
Temperature and concentration control of exothermic chemical processes in continuous stirred tank reactors
Exothermic chemical reaction taking place in continuous stirred tank reactor is considered. Heat release from the chemical reaction, non-linear dynamic behavior of the process and uncertainty in parameters are the main factors motivating the use of robust control design. Viewing temperature and molar concentration as variables both accessible in real time, PI and optimal state-feedback controllers driven by temperature and concentration error signals are proposed to regulate the system over reactorâs steady-state working points by counteracting undesired disturbances. Since access to concentration value has proved beneficial for the reactorâs performance, estimation techniques are examined to compensate for the problematic nature of the concentrationâs measurement. A linear reduced-order observer is first proposed to estimate the concentration value using temperature measurements. In addition, assuming concentration measurement is available with a relatively short delay via sample analysis, a linear and non-linear discrete-time predictor is constructed to estimate the concentrationâs real-time value. A linear combination of the two estimation schemes (observer, predictor) is proposed resulting in a combined estimator, in which the emphasis between the two individual schemes can be controlled via a scalar parameter. The work presented in this paper was supported by the GLOW project â New weather-stable low gloss powder coatings based on bifunctional acrylic solid resins and nanoadditives â as part of the development of novel and efficient processing technologies regarding the production of new families of powder coatings, responding to industrial requirements for quality improvement at lower cost and shorter development cycles
Overdensity of SMGs in fields containing z ⌠0.3 galaxies: magnification bias and the implications for studies of galaxy evolution
We report a remarkable overdensity of high-redshift submillimetre galaxies (SMG), 4â7 times the background, around a statistically complete sample of twelve 250âÎŒmâselected galaxies at z = 0.35, which were targeted by ALMA in a study of gas tracers. This overdensity is consistent with the effect of lensing by the haloes hosting the target z = 0.35 galaxies. The angular cross-correlation in this sample is consistent with statistical measures of this effect made using larger sub-mm samples. The magnitude of the overdensity as a function of radial separation is consistent with intermediate scale lensing by haloes of the order of 7Ă1013 Mâ â , which should host one or possibly two bright galaxies and several smaller satellites. This is supported by observational evidence of interaction with satellites in four out of the six fields with SMG, and membership of a spectroscopically defined group for a fifth. We also investigate the impact of these SMG on the reported Herschel fluxes of the z = 0.35 galaxies, as they produce significant contamination in the 350 and 500âÎŒmâHerschel bands. The higher than random incidence of these boosting events implies a significantly larger bias in the sub-mm colours of Herschel sources associated with z < 0.7 galaxies than has previously been assumed, with fboost = 1.13, 1.26, 1.44 at 250, 350, and 500âÎŒmâ. This could have implications for studies of spectral energy distributions, source counts, and luminosity functions based on Herschel samples at z = 0.2â0.7
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