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Magnetic Bubbles and Jets: Episodic Phenomena in Astrophysical Jets

In their first million years or so, forming stars accrete their mass and emerge from their parent molecular cloud as newly born stars. During these early stages of evolution, young (proto-)stars eject powerful streams of plasma (jets) that reach distances of several parsecs (1 pc ~ 200 000 AU ~ 3 x 1013 km). Jets are also observed in many other systems (galaxies, planetary nebulae, supernovae, ...) and are thought to originate from the same mechanism: rotating compact sources (star-disk) producing a highly twisted magnetic field which accelerates and collimates the flow.

Figure 1 : schematic diagram of the magnetic bubbles and jets formation.

Here we report on the first experimental simulations exploring episodic, magnetohydrodynamic (MHD) jets. A schematic diagram of the experiments is shown in figure 1. Two components are generally present: a magnetic bubble (or cavity) accelerated by gradients of the magnetic pressure and a magnetically confined jet on its interior. Typical flow velocities measured in the experiments are ~ 100 - 400 km/s, and jet temperatures of up to ~ 106 K have been also observed.Similarly to astrophysical jets, laboratory jets are confined by a toroidal magnetic field, in a configuration that is known to be highly unstable, and which may destroy the flow. However the experiments show the possible role played by these instabilities in the evolution of astrophysical jets: rather than disrupting the entire outflow, the end result is the formation of a highly collimated, clumpy stream of " plasmoids ", which continue to propagate ballistically detaching from the acceleration region (the source). Furthermore the instabilities lead to the generation of a tangled magnetic field which helps to collimate successive bubbles. An important aspect of the episodic ejection process investigated is, broadly speaking, its self-collimation. The initial ambient medium is swept away after a few ejections, and newly formed magnetic cavities are confined solely by the environment (plasma and magnetic field) left by earlier episodes, thus making the long-term collimation insensitive to the initial ambient conditions.

The figure 2 shows on the left the MHD simulations of the experiments showing the twisted magnetic field lines. A newly formed bubble can be seen emerging through the gap in the foil. the figure on the right shows the experimental, filtered XUV emission with two nested magnetic bubbles and a jet.

Figure 2 : 3D MHD simulations of the experiment and instantaneous XUV emission of the experimental bubbles.

By " scaling " the dynamics observed in the experiments a qualitative view of the evolution of astrophysical jets emerges: the presence of multiple bubble-like features should be observed on scales ranging from a few tens to a few hundred AU from the source, and because of the relatively short growth time of the instabilities, astrophysical jets should develop non-axisymmetric features already within a few tens AU from the source; becoming more heterogeneous and clumpy as they move further away to hundreds of AU. Over the same length scales we would expect magnetic energy dissipation, heating of the plasma and a transition to a kinetically dominated jet which propagates ballistically.

A compelling astrophysical system which shows many of these features is the jet from the young star DG Tau. Ejection variability, limb-brightened bubble-like structures, the presence of wiggles and considerable asymmetries with respect to the axis are evident on scales ranging from of a few tens to a few hundred AU the source, the experiments clearly demonstrate that asymmetries in the flow can be produced by instabilities that do not destroy the collimation. The flow in XZ Tau also clearly shows episodic activity in the form of rapidly evolving " bubbles " with a clumpy jet-like morphology along the axis (figure 2).

Figure 2: X Ztau bubbles ejections, observed at HST (John Krist (STScI) et al., WFPC2, HST, NASA).

It was also recently reported for a number of T-Tauri jets including DG Tau, that already within 100 AU from the source the jet physical conditions show considerable asymmetries with respect to the axis, the experiments clearly demonstrate that asymmetries in the flow can be produced by instabilities that do not destroy the collimation. Furthermore X-ray emission from the DG Tau jet was recently detected on the same length scales and it was suggested that magnetic energy dissipation may be behind the heating mechanism. As in the experiments, tangled magnetic fields produced by the instability may provide a compelling route to plasma heating.

In summary, two key principles emerge from the study: first, toroidal fields can collimate and accelerate flows to super-magnetosonic speeds. Second, even if the average flow geometry and collimation is " steady " over long time-scales, the jet activity can be episodic and the instabilities need not disrupt the long term collimation. The results offer a first glimpse on how magnetohydrodynamic outflows may naturally evolve from a relatively steady-state launching to a heterogeneous jet, suggesting a new scenario which shifts emphasis away from stationary, steady-state production of continuous jet beams, towards an episodic, ejection of plasmoids surrounded by evolving magnetic field configurations.


"Episodic Magnetic Bubbles and Jets: Astrophysical Implications from Laboratory Experiments",
A. Ciardi, S.V. Lebedev, A. Frank, F. Suzuki-Vidal, G.N. Hall, S.N. Bland, A. Harvey-Thompson, E.G. Blackman, M. Camenzind,
The Astrophysical Journal Letters, Volume 691, Issue 2, pp. L147-L150 (2009), arXiv:0811.2736, Reprint

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