Stellar jets: comparison between laboratory experiment and numerical simulations |
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Stellar jets: comparison between laboratory experiment and numerical simulations
Researchers from the European network JETSET, including astronomers from Paris Observatory, have just studied the jets produced by young stars ejecting matter, the Herbig Haro objects. For this study they reproduced these jets, on one hand by laboratory experiments, on the other hand by numerical simulations. This double, experimental and virtual, approach allows to show that the interstellar wind plays a fundamental role while interacting with the jet, by creating these nodules and these structure breaks.
The Herbig Haro (HH) objects are nebulosities associated with young stars. They correspond to matter ejection in the shape of jets. These jets present on their way a complicated succession of bow shocks and nodes and a complex and variable dynamics which results from their interaction with the interstellar medium (cf HH 47, Figure 1). Their morphology is a tracer of the history of their formation and of their interaction with the interstellar medium. Certain jets, for example HH 502, have a curve in a C shape which is usually attributed to the presence of an "effective" wind produced, either by the own movement of the source in the interstellar medium, or by flows resulting from nearby sources. Figure 1: Image of the Herbig Haro 47 jet, taken with the Space Telescope. © HST. NAS/ESA.
Click on the image to enlarge itTo study the dynamics and the structure of these jets and the origin of these curves, the international team of researchers has just carried out a double approach: the experiment in laboratory and the numerical simulation. As regards the experiment in laboratory, the experimental jets were produced on the generator of pulsated current MAGPIE of the Imperial College. The experimental jet has the required characteristics of HH jets in young stars, but with time scales and distances different, of the order of nanoseconds and centimetres. The typical propagation velocity is about 100 to 200 km/s. The interaction of this experimental jet with a lateral wind (30 to 50 km/s) is obtained thanks to a ablation plasma generated on a sheet by a strong XUV radiation. With regard to the numerical simulation, the virtual astrophysical jet was obtained with typical parameters of the stellar jets, for example with flow times of hundreds of years and distances equivalent to hundreds of astronomical units (an astronomical unit = 150 million km).
Figure 2: Design of the laboratory experiment (Left) Emission XUV from the experiment which reveals an internal shock and a curved jet (Right). © LERMA. SNL. BLIC.
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The dynamics of the jet is similar in the experiment and the simulation, marked by the formation of a series of bow shocks and nodes. The collision between the jet and the wind generates a bow shock which wraps all the jet. The jet develops a very asymmetrical form and the upstream part of the cocoon disappears. An oblique shock is formed in the center of the jet and starts to curve the latter. The head of the jet ends up being detached and its movement becomes ballistic. The presence of internal shocks is visible at the same time in the experiment and the simulation. An important result is that the structure observed in the "pseudo HH jet" and the destruction of the bow shock result only from the interaction with the wind and are not related to some variability of the injection of the jet. It is the same for the internal shocks. The jet is prone to instabilities which tend to separate the jet in filaments.
Figure 3: Simulation of an astrophysical jet (100 km/s, 1 000 particules per cm3) interacting with a latéral wind ( 25 km/s, 100 particules per cm3 ). The scale is 2 004 x 4 864 astroomical units. © LERMA. SNL. BLIC.
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Reference
A. Ciardi (LERMA, Meudon), D.J. Ampleford (Albuquerque, USA), S.V. Lebedev (London, UK), C. Stehle (LERMA, Meudon)
Curved Herbig-Haro Jets: Simulations and Experiments
Preprint in Astrophysical JournalContact
Andréa Ciardi (Observatoire de Paris, LERMA et CNRS)
Chantal Stehlé (Observatoire de Paris, LERMA, et CNRS)
