267 research outputs found
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
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
On the magnetic acceleration and collimation of astrophysical outflows
The axisymmetric 3-D MHD outflow of a cold plasma from a magnetized and
rotating astrophysical object is numerically simulated with the purpose of
investigating the outflow's magnetocentrifugal acceleration and eventual
collimation. Gravity and thermal pressure are neglected while a split-monopole
is used to describe the initial magnetic field configuration. It is found that
the stationary final state depends critically on a single parameter alpha
expressing the ratio of the corotating speed at the Alfven distance to the
initial flow speed along the initial monopole-like magnetic fieldlines. Several
angular velocity laws have been used for relativistic and nonrelativistic
outflows. The acceleration of the flow is most effective at the equatorial
plane and the terminal flow speed depends linearly on alpha. Significant flow
collimation is found in nonrelativistic efficient magnetic rotators
corresponding to relatively larger than 1 values of alpha while very weak
collimation occurs in inefficient magnetic rotators with values of alpha
smaller than about 1. Part of the flow around the rotation and magnetic axis is
cylindrically collimated while the remaining part obtains radial asymptotics.
The transverse radius of the jet is inversely proportional to alpha while the
density in the jet grows linearly with alpha. For alpha greater than about 5
the magnitude of the flow in the jet remains below the fast MHD wave speed
everywhere. In relativistic outflows, no collimation is found in the supersonic
region for parameters typical for radio pulsars. All above results verify the
main conclusions of general theoretical studies on the magnetic acceleration
and collimation of outflows from magnetic rotators and extend previous
numerical simulations to large stellar distances.Comment: 15 pages, 13 figures. Accepted for publication, MNRA
MHD models and synthetic synchrotron maps for the jet of M87
We present a self-consistent MHD model 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 for a wide range of parameters and matches the measurements of the opening
angle of M87 over several orders of magnitude in spatial extent.
In the second part of this work, we present synthetic synchrotron emission
maps for our MHD models. In principle the two-zone model can reproduce the
morphological structure seen in radio observations, as central-peaked profiles
across the jet close the the source, limb-bright further down the jet, and a
bright knot close to the position of HST-1. However it is difficult to
reconcile all features into a single set of parameters.Comment: 4 pages, 5 figures, to appear in the proceedings of the HEPRO
conference, September 24-28, 2007, Dublin, Irelan
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
Multiple transonic solutions and a new class of shock transitions in solar and stellar winds
The steady isothermal solar wind equations are shown to admit, under certain circumstances, mutliple transonic solutions when, for example, momentum deposition gives rise to multiplee critical points in the flow. These multiple solutions consist of a continuous solution and solutions which involve shock transitions between critical solutions. The ambiguity arising from the multiplicity of the solutions can be resolved by following the time evolution of a wind profile with one critical point. Results of the numerical integration of the time-dependent equations with momentum addition show that each of these multiple solutions is physically accessible and depends on the rate of change of momentum deposition. These results suggest that standing shocks are likely to be present in the inner solar wind flow
Formation of standing shocks in stellar winds and related astrophysical flows
Stellar winds and other analogous astrophysical flows can be described, to lowest order, by the familiar one dimensional hydrodynamic equations which, being nonlinear, admit in some instances discontinuous as well as continuous transonic solutions for identical inner boundary conditions. The characteristics of the time dependent differential equations of motion are described to show how a perturbation changes profile in time and, under well defined conditions, develops into a stationary shock discontinuity. The formation of standing shocks in wind type astrophysical flows depends on the fulfillment of appropriate necessary conditions, which are determined by the conservation of mass, momentum and energy across the discontinuity, and certain sufficient conditions, which are determined by the flow's history
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