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Curved jets in space and in the laboratory

Herbig-Haro (HH) jets are one of the most spectacular phenomena associated with star formation. Jets generally show an intricate succession of bow shocks and knots along their flow, and a complex and rich dynamics arising from the interaction with the circumstellar environment. The morphology of jets traces both the history of their generation and that of the interaction with the interstellar medium (ISM). For instance, the C-shape morphology that is observed in many HH jets is usually associated with the presence of an "effective" wind, produced either by the proper motion of the jet source through the ISM or by outflows from close-by sources (Figure 1).

Figure 1: HH 502 jet in the Orion Nebula seen in Halpha (rigth) and [SII] (left), (from Bally and Reipurth, ApJ 2001)
HH502

Bent jets are also observed in the laboratory under conditions directly relevant to the astrophysical jets. The similarity is a consequence of certain transformation properties of the inviscid fluid equations and is obtained through a careful scaling of the experimental plasma conditions. Astrophysically relevant laboratory jets are produced on MAGPIE pulsed-power generator using a conical cage of micron-sized metallic (tungsten, aluminium, etc.) wires. The radiatively cooled laboratory jets have high Mach numbers (M ~ 20 - 40) and Reynolds numbers (Re > 10^4); typical flow velocities are of the order of 100-200 km/s. The interaction of the jet with a side-wind (~ 30-ˆ’50 km/s) is reproduced by placing a plastic foil in the jet propagation region (Lebedev et al., ApJ 2004)

schematic
Figure 2: The experimental setup showing a conical wire array. The wind is produced by radiative ablation. The source of the radiation is the hot plasma (30-50 eV) present in the conical shock that is generated on the axis of the array by the converging plasma streams.

To study the dynamics of astrophysical curved jets in the laboratory astrophysics context, we took a global and novel approach where we performed not only the experiments and the numerical simulations of the experiments, but also the numerical modelling of scaled up astrophysical jets. For the simulated HH jet-wind interaction we used jet and wind parameters characteristic of the astrophysical environment. However we also took the dimensionless parameters relevant to the interaction, such as the ratio of the jet and wind momentum fluxes, to be similar to those of the laboratory experiments.

The jet-wind collision generates a bow shock which envelopes the whole jet. As the bow shock is advected downstream with the wind, it develops a highly asymmetric shape, with the upwind side of the jet cocoon effectively disappearing. An oblique shock forms in the jet body which begins to bend the jet. As the jet curves, momentum transferred from the jet to the working surface rapidly decreases, until the working surface effectively detaches from the jet. The subsequent propagation of the head of the jet is ballistic. The presence of internal shocks in the jet beam is visible in both the simulations and experiment (figure 3)

expjet
Figure 3: Experimental jet seen in XUV emission: internal shocks are clearly visible.

The astrophysical jet-wind dynamics is simulated using an appropriately modified version of our laboratory code. Despite of the different time and length scales (several hundred years compared to several hundred nanoseconds, and thousands of AU compared to a few cm) the overall jet dynamics is similar in both systems, and it is marked by the formation of a number of bow-shaped shocks and knots.

One important results is that the clumpiness seen in the HH jets and the break-up of the bow shock arise entirely from the interaction with the wind and are not imposed by means of a variable jet injection velocity. The internal shocks are a dynamical observable feature and may thus in some cases depend on interaction with the environment and not exclusively on a variable jet velocity profile (figure 4).

Figure 4: simulated astrophysical bent jet
simulated jet

Another important result is that the jet is liable to the growth of the Rayleigh-Taylor instability (RTI) which tends to split the jet into well defined filaments.
Models and observations suggest that tellar jets are rotating, and we examined the effect of the rotation of the jet on its dynamics. In the simulations, the jet is taken to rotate as a solid body and we set the azimuthal velocity at its boundary to 0.5cs, where cs is the initial sound speed in the jet. Although exhibiting an overall similar global topology, the rotating jets tend to have a slightly higher curvature, which is essentially due to the resulting larger diameter produced by rotation. However the main effect of rotation on the jet-wind interaction dynamics is that it reduces the growth of the RTI and the disruption of the jet. The rotation effectively shears the growing RT modes confining them to a narrow layer of the jet body. It is then the combination of RTI and the Kelvin-Helmholtz instability that disrupts the rotating jet. Observing the full development of the instabilities in the laboratory jets will require longer interaction times (> 200 ns) than currently obtained (~ 100-150 ns). Such experiments, also with the inclusion of rotation, are planned for the future.

Reference

"Curved Herbig-Haro Jets: Simulations and Experiments",
A. Ciardi, D.J. Ampleford, S.V. Lebedev, C. Stehlé,
Astrophysical Journal (accepted), arXiv:0712.0959, Reprint

English
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