Jets are magnetic!

New simulations confirm the theory whereby astrophysical jets are produced by magnetic fields. The calculations cover a larger range than ever before. They show how magnetic instabilities create structures which share a large similarity with observations of protostellar jets.

Fig. 1: Magnetic field lines in a jet. The helical twist of the field is responsible both for the acceleration of the gas and the instability of the jet. In the upper part of the left-hand image, one can see instabilities in the form of kink-like perturbations. The right-hand image shows an expanded view of the lower part, where the field line structure is still ordered.

Fig. 2: This plot shows what a simulated jet might look like when observed from Earth. Bright regions indicate high temperature and high magnetic field strength. These would also appear bright in the observations. (linkPfeil.gifMovie: temporal evolution.)

Jet-shaped outflows of hot gas are quite common in the Universe. Well-known examples can be found in so-called "active" galaxies, in whose cores jets are formed and then travel enormous distances. The length of the jet can be a billion times the size of the core region in which it forms. The core region itself consists of a supermassive black hole which devours matter through a so-called accretion disk, getting even more massive in the process. A similar situation can be found in protostars, where a new star grows out of a hydrogen cloud, with the difference that the central object is not a black hole, but the nascent star. Jets from such objects also cover enormous distances of about a million times the distance between the Earth and the Sun. Finally, there are jets which cannot be directly observed, but which are linked to momentary bursts of short-wave radiation, the so-called gamma ray bursts. Many of these events are probably caused by a black hole with an accretion disk in the core of a supermassive, rapidly rotating star at the end of its life.

A crucial role in the formation of jets is probably played by magnetic fields that are anchored in the rotating central object, e.g. the abovementioned accretion disk. The magnetic field gets helically twisted by the rotation, inducing a force that accelerates the matter within the jet. However, such jets are potentially unstable: small "kinks" in the jet tend to become larger with time. This can go as far as destroying the jet. Basically the same problem appears in experiments of nuclear fusion.

With the new calculations, it was possible to reproduce jets in simulations and follow their evolution up to a large distance from the source. It turned out that jets are not necessarily destroyed by the instabilities. Rather, wiggly structures are formed which are also seen in observations of real jets. Part of the magnetic field gets dissipated, i.e. it is "destroyed" and the energy stored in the field is converted into heat. Such hot regions would appear as bright knots in the observations.

The calculations have shown that for a simulation to be realistic the jet has to be followed over a large distance. This requires the use of powerful computers for a long time, a use of huge amounts of resources. With continuing increases in computing power, there will be opportunities for even bigger and better jet simulations.


Rainer Moll


Further Information:

R. Moll, H. C. Spruit & M. Obergaulinger, "Kink instabilities in jets from rotating magnetic fields", in Astronomy & Astrophysics, volume 492, pp. 621--630