Researchers Explain Formation of Jets Emitted by Young Stars

Using a patented experimental device and supercomputer simulations, researchers have managed to explain the formation of jets emitted by young stars. In perfect agreement with astrophysical observations, the model, which involves the interstellar magnetic field, was developed by an international collaboration¹ led by French teams at the Laboratoire pour l'Utilisation des Lasers Intenses (LULI, CNRS/École Polytechnique/UPMC/CEA), the Laboratoire d'Etudes du Rayonnement et de la Matière en Astrophysique et atmosphères (LERMA, Observatoire de Paris/CNRS/UPMC/Université de Cergy-Pontoise/ENS Paris) and the Laboratoire National des Champs Magnétiques Intenses (LNCMI, CNRS). Their work is published in the 17 October 2014 issue of the journal Science.

Astrophysical jets are narrow streams of matter that can propagate over huge distances (hundreds of times the distance from the Earth to the Sun). They are found throughout the Universe, emerging from objects as diverse as stars in formation, white dwarfs, neutron stars and black holes. All these systems have one thing in common: they are in the active process of accreting matter from their immediate environment via a rotating disc of matter. Although spectacular, such jets were long seen as mere by-products of the matter accretion process. However, physicists gradually began to realise that they actually play a key role in this phenomenon. For instance, the jets emitted from the poles of a newly born star slow down the rotation of the gas collapsing onto the central core, enabling matter to continue to be accreted. In addition, their action on the interstellar medium can lead to the birth of new stars. However, despite their importance, jets remain one of the most puzzling phenomena in modern astronomy. In particular, current theories fail to explain how matter can propagate over such long distances while remaining confined in a narrow jet.

Thanks to the first ever simulation of this phenomenon in the laboratory, and to three-dimensional numerical modeling, the researchers have discovered that the jets emitted by very young stars are confined by a large-scale magnetic field aligned with the axis of the jets, as recently shown by telescope measurements. The mechanism proposed by the researchers is thus in very good agreement with current astrophysical observations. In particular, it helps to explain the puzzling X-ray emissions observed along the jet flow by the Chandra satellite.

Lasers and coils used to reproduce the interstellar environment

These results were made possible by a recently patented novel experimental set-up that combines powerful lasers with ultra-strong magnetic fields. By using laser beams to extract electrons from a sample of plastic, physicists at LULI produced a plasma² that was a scaled down version of the atmosphere of young stars. However, the determining factor was the use of coils produced at LNCMI to generate a magnetic field that was powerful enough to reproduce, in just a few cubic centimeters and for a few millionths of a second, the interstellar environment. Physicists at LERMA and LULI, helped by collaborators from other countries, then used supercomputers to model young stars in formation and the laboratory experiment. The agreement between both simulations confirmed the key role played by the interstellar magnetic field.

In this study, the scientists specifically focused on plasma jets in newly born stars. However, the same mechanism may be at work in other types of astrophysical jets. In addition, this work opens the way to the practical study of the role of magnetic fields in astrophysics. The researchers intend to focus on the mechanism of accretion of matter by young stars, on cosmic rays³, and on the arches of plasma ejected during solar flares. Lastly, the device built at LULI could be used for research into nuclear fusion, where it has long been known that magnetic fields are needed to confine the ions within the fuel and increase their temperature, which is the key parameter required to bring about fusion.

¹ This research involved scientists from France, Italy, Germany, the UK, Russia, Japan, the US and Canada.
² A plasma is an ionized gas made up of ions and free electrons.
³ Cosmic rays is the term used to describe a flow of very high energy charged particles arriving from space, whose exact origin is still unknown.

Source: http://www.cnrs.fr/