The Propagation of Magneto-Centrifugally Launched Jets: I

We present simulations of the propagation of magnetized jets. This work differs from previous studies in that the cross-sectional distributions of the jets's state variables are derived from analytical models for magneto-centrifugal launching. The source is a magnetized rotator whose properties are specfied as boundary conditions. The jets in these simulations are considerably more complex than the ``top-hat''constant density etc. profiles used in previous work. We find that density and magnetic field stratification (with radius) in the jet leads to new behavior including the separation of an inner jet core from a low density collar. We find this {\it jet within a jet} structure, along with the magnetic stresses, leads to propagation behaviors not observed in previous simulation studies. Our methodology allows us to compare MHD jets from different types of sources whose properties could ultimately be derived from the behavior of the propagating jets.Comment: 42 pages, accepted by the Ap

Plasma physics in clusters of galaxies

Clusters of galaxies are the largest self-gravitating structures in the universe. Each cluster is filled with a large-scale plasma atmosphere, in which primordial matter is mixed with matter that has been processed inside stars. This is a wonderful plasma physics laboratory. Our diagnostics are the data we obtain from X-ray and radio telescopes. The thermal plasma is a strong X-ray source; from this we determine its density and temperature. Radio data reveal a relativistic component in the plasma, and first measurements of the intracluster magnetic field have now been made. Energization of the particles and the field must be related to the cosmological evolution of the cluster. The situation is made even richer by the few galaxies in each cluster which host radio jets. In these galaxies, electrodynamics near a massive black hole in the core of the galaxy lead to a collimated plasma beam which propagates from the nucleus out to supergalactic scales. These jets interact with the cluster plasma to form the structures known as radio galaxies. The interaction disturbs and energizes the cluster plasma. This complicates the story but also helps us understand both the radio jets and the cluster plasma.Comment: 12 pages, 6 figures, 3 in color. Invited review, to appear in Physics of Plasmas, May 2003. After publication it can be found at http://ojps.aip.org/po

Structure and Stability of Keplerian MHD Jets

MHD jet equilibria that depend on source properties are obtained using a simplified model for stationary, axisymmetric and rotating magnetized outflows. The present rotation laws are more complex than previously considered and include a Keplerian disc. The ensuing jets have a dense, current-carrying central core surrounded by an outer collar with a return current. The intermediate part of the jet is almost current-free and is magnetically dominated. Most of the momentum is located around the axis in the dense core and this region is likely to dominate the dynamics of the jet. We address the linear stability and the non-linear development of instabilities for our models using both analytical and 2.5-D numerical simulation's. The instabilities seen in the simulations develop with a wavelength and growth time that are well matched by the stability analysis. The modes explored in this work may provide a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap

Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds

Two three-dimensional magnetohydrodynamical simulations of strongly magnetized conical jets, one with a poloidal and one with a helical magnetic field, have been performed. In the poloidal simulation a significant sheath (wind) of magnetized moving material developed and partially stabilized the jet to helical twisting. The fundamental pinch mode was not similarly affected and emission knots developed in the poloidal simulation. Thus, astrophysical jets surrounded by outflowing winds could develop knotty structures along a straight jet triggered by pinching. Where helical twisting dominated the dynamics, magnetic field orientation along the line-of-sight could be organized by the toroidal flow field accompanying helical twisting. On astrophysical jets such structure could lead to a reversal of the direction of Faraday rotation in adjacent zones along a jet. Theoretical analysis showed that the different dynamical behavior of the two simulations could be entirely understood as a result of dependence on the velocity shear between jet and wind which must exceed a surface Alfven speed before the jet becomes unstable to helical and higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September

Transit flow models for low and high mass protostars

In this work, the gas infall and the formation of outflows around low and high mass protostars are investigated. A radial self-similar approach to model the transit of the molecular gas around the central object is employed. We include gravitational and radiative fields to produce heated pressure-driven outflows with magneto-centrifugal acceleration and collimation. Outflow solutions with negligible or vanishing magnetic field are reported. They indicate that thermodynamics is a sufficient engine to generate an outflow. The magnetized solutions show dynamically significant differences in the axial region, precisely where the radial velocity and collimation are the largest. They compare quantitatively well with observations. The influence of the opacity on the transit solutions is also studied. It is found that, when dust is not the dominant coolant, such as in the primordial universe, mass infall rates have substantial larger values in the equatorial region. This suggests that star forming in a dust-free environment should be able to accrete much more mass and become more massive than present day protostars.It is also suggested that molecular outflows may be dominated by the global transit of material around the protostar during the very early stages of star formation, especially in the case of massive or dust-free star formation.Comment: 19 pages, 15 figures, accepted by Ap

Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets

The linearized relativistic magnetohydrodynamic (RMHD) equations describing a uniform axially magnetized cylindrical relativistic jet spine embedded in a uniform axially magnetized relativistically moving sheath are derived. The displacement current is retained in the equations so that effects associated with Alfven wave propagation near light speed can be studied. A dispersion relation for the normal modes is obtained. Analytical solutions for the normal modes in the low and high frequency limits are found and a general stability condition is determined. A trans-Alfvenic and even a super-Alfvenic relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz modes. The resonance condition for maximum growth of the normal modes is obtained in the kinetically and magnetically dominated regimes. Numerical solution of the dispersion relation verifies the analytical solutions and is used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap

A Global Jet/Circulation Model for Young Stars

Powerful, highly collimated jets, surrounded by bipolar molecular outflows, are commonly observed near Young Stellar Objects (YSOs). In the usual theoretical picture of star formation, a jet is ejected from a magnetized accretion disk, with a molecular outflow being driven either by the jet or by a wider wind coming from the disk. Here, we propose an alternative global model for the flows surrounding YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infalling matter and follows a circulation pattern around the central object without necessarily being entrained by a jet. It is shown that the model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We report solutions for the three different parts of this self-similar model, i.e. the jet, the infalling envelope and the circulating matter that eventually forms the molecular outflow. This new picture of the accretion/outflow phase provides a possible explanation for several observed properties of YSO outflows. The most relevant ones are the presence of high mass molecular outflows around massive protostars, and a realistic fraction (typically 0.1) of the accretion flow that goes into the jet.Comment: accepted for publication in Astronomy and Astrophysic