410 research outputs found
A class of exact MHD models for astrophysical jets
This paper examines a new class of exact and self-consistent MHD solutions
which describe steady and axisymmetric hydromagnetic outflows from the
atmosphere of a magnetized and rotating central object with possibly an
orbiting accretion disk. The plasma is driven against gravity by a thermal
pressure gradient, as well as by magnetic rotator and radiative forces. At the
Alfvenic and fast critical points the appropriate criticality conditions are
applied. The outflow starts almost radially but after the Alfven transition and
before the fast critical surface is encountered the magnetic pinching force
bends the poloidal streamlines into a cylindrical jet-type shape. The terminal
speed, Alfven number, cross-sectional area of the jet, as well as its final
pressure and density obtain uniform values at large distances from the source.
The goal of the study is to give an analytical discussion of the
two-dimensional interplay of the thermal pressure gradient, gravitational,
Lorentz and inertial forces in accelerating and collimating an MHD flow. A
parametric study of the model is also given, as well as a brief sketch of its
applicability to a self-consistent modelling of collimated outflows from
various astrophysical objects. {The analysed model succeeds to give for the
first time an exact and self-consistent MHD solution for jet-type outflows
extending from the stellar surface to infinity where it can be superfast, in
agreement with the MHD causality principle.Comment: 16 pages, 15 figures. Accepted for publication in MNRA
Systematic construction of exact MHD models for astrophysical winds and jets
By a systematic method we construct general classes of exact and
selfconsistent axisymmetric MHD solutions describing flows which originate at
the near environment of a central gravitating astrophysical object. The
unifying scheme contains two large groups of exact MHD outflow models, (I)
meridionally self-similar ones with spherical critical surfaces and (II)
radially self-similar models with conical critical surfaces. The classification
includes known polytropic models, such as the classical Park er model of a
stellar wind and the Blandford and Payne (1982) model of a disk-wind; it also
contains nonpolytropic models, such as those of winds/jets in Sauty and
Tsinganos (1994), Lima et al (1996) and Trussoni et al (1997). Besides the
unification of these known cases under a common scheme, several new classes
emerge and some are briefly analysed; they could be explored for a further
understanding of the physical properties of MHD outflows from various
magnetized and rotating astrophysical objects in stellar or galactic systems.Comment: 13 pages, 11 figure
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
Relativistic expansion of a magnetized fluid
We study semi-analytical time-dependent solutions of the relativistic
magnetohydrodynamic (MHD) equations for the fields and the fluid emerging from
a spherical source. We assume uniform expansion of the field and the fluid and
a polytropic relation between the density and the pressure of the fluid. The
expansion velocity is small near the base but approaches the speed of light at
the light sphere where the flux terminates. We find self-consistent solutions
for the density and the magnetic flux. The details of the solution depend on
the ratio of the toroidal and the poloidal magnetic field, the ratio of the
energy carried by the fluid and the electromagnetic field and the maximum
velocity it reaches.Comment: 17 pages, 6 figures, accepted by Geophysical and Astrophysical Fluid
Dynamic
Steady-state rarefaction waves in relativistic magnetized flows: Theory and application to gamma-ray burst outflows
We investigate the characteristics of a relativistic magnetized fluid flowing around a corner. If the outflow is faster than the fast-magnetosonic speed (or sound speed for a non-magnetized fluid) the non-smooth boundary induces a rarefaction wave propagating in the body of the flow. The subsequent expansion is accompanied with a very efficient increase of the flow bulk speed and Lorentz factor. We apply this “rarefaction acceleration mechanism” to the Collapsar model of gamma-ray bursts, in which a relativistic jet initially propagates in the interior of the progenitor star, before crossing the stellar surface with a simultaneous drop in the external pressure support. We integrated the steady-state equations using a special set of partial solutions, called r – self similar. The use of these solutions degrades the system of the complex, non-linear, 2nd order partial differential equations into a system of two 1st order ordinary differential equations whose integration is straightforward. For the conditions expected in a GRB, a fully analytical solution can also be obtained. The aim of this work is to give insight to the results of recent time-depended numerical simulations and show that rarefaction is a plausible mechanism for these phenomena
A radio jet drives a molecular and atomic gas outflow in multiple regions within one square kiloparsec of the nucleus of the nearby galaxy IC5063
We analyzed near-infrared data of the nearby galaxy IC5063 taken with the
Very Large Telescope SINFONI instrument. IC5063 is an elliptical galaxy that
has a radio jet nearly aligned with the major axis of a gas disk in its center.
The data reveal multiple signatures of molecular and atomic gas that has been
kinematically distorted by the passage of the jet plasma or cocoon within an
area of ~1 kpc^2. Concrete evidence that the interaction of the jet with the
gas causes the gas to accelerate comes from the detection of outflows in four
different regions along the jet trail: near the two radio lobes, between the
radio emission tip and the optical narrow-line-region cone, and at a region
with diffuse 17.8 GHz emission midway between the nucleus and the north radio
lobe. The outflow in the latter region is biconical, centered 240 pc away from
the nucleus, and oriented perpendicularly to the jet trail. The diffuse
emission that is observed as a result of the gas entrainment or scattering
unfolds around the trail and away from the nucleus with increasing velocity. It
overall extends for >700 pc parallel and perpendicular to the trail. Near the
outflow starting points, the gas has a velocity excess of 600 km/s to 1200 km/s
with respect to ordered motions, as seen in [FeII], Pa alpha, or H2 lines. High
H2 (1-0) S(3)/S(1) flux ratios indicate non-thermal excitation of gas in the
diffuse outflow.Comment: Accepted for publication in Ap
Steady-state rarefaction waves in magnetized flows and their application to gamma-ray burst outflows
We investigate the characteristics of a relativistic magnetized fluid flowing around a corner. If the flow is faster than the fast-magnetosonic speed the non-smooth boundary induces a rarefaction wave propagating in the body of the flow. The subsequent expansion is accompanied with a very efficient increase of the flow speed and bulk Lorentz factor γ. We apply this ”rarefaction acceleration mechanism” to the collapsar model of gamma-ray bursts, in which a relativistic jet initially propagates in the interior of the progenitor star, before crossing the stellar surface with a simultaneous drop in the external pressure support. We integrate the steady-state equations using a special set of partial (r self-similar) solutions. The use of these solutions degrades the system of the complex, non-linear, 2nd order partial differential equations into a system of two 1st order ordinary differential equations whose integration is straightforward. For the conditions expected in a gamma-ray burst, a fully analytical solution can be obtained. The aim of this work is to better understand the results of recent timedepended numerical simulations and show that rarefaction acceleration is a plausible mechanism in gamma-ray burst outflows
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
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
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