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

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

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

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