Stellar Jets

Linear streams of matter arising from a number of different sources (T Tauri stars, planetary nebulae, and compact objects). Stellar jets from T Tauri stars are common in star forming regions. These are called Herbig-Haro objects (q.v.). As material falls onto a protostar from the surrounding accretion disk, interactions between the magnetic fields of the rotating star and the accretion disk eject it from the stellar magnetic poles. However, this accretion process is not smooth, and there may be sudden increases or decreases in the rate of accretion onto the protostar. A sudden variations in the accretion rate produce "bullets" of denser material in the jets (HH34).

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Stellar jets interact with the surrounding interstellar medium. T Tauri stars occur in star forming nebulae and their jets interact with gas remaining from the formation of the star itself, effectively sweeping out the interstellar material in their path. The swept out material accumulates at the head of the jet causing increased resistance. This eventually forms a shock front, with the jet having to force its way through the accumulated gas and dust, ionizing it.
It is not only young stars that exhibit stellar jets. Jets are also observed in planetary nebulae, which form during the final stages of evolution of stars similar to (or a little more massive than) the Sun. The jets of planetary nebulae have speeds similar to those of T Tauri jets, but are generally less well collimated (broader for their length). They form when material flowing out of the star is diverted into a jet. The collimation is usually caused by a confining equatorial disk that blocks equatorial outflow and directs the material into polar jets. These disks are often found in binary systems. For example, the star at the heart of the planetary nebula M2-9 (below) is a spectroscopic binary (a pair of stars orbiting so close to each other that we are unable to separate them in images). The closeness of the two stars causes a ring of gas to be thrown off. This encircles the pair of stars, blocking the equatorial ejection of gas from the dying star and diverting it into polar outflows.

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The last sources of stellar jets are compact objects (stellar black holes and neutron stars) in binary systems. The compact object is surrounded by an accretion disk formed through Roche-lobe overflow from a companion giant star. As is the case with T Tauri jets, the jets form as material falls onto the compact object and is ejected from its magnetic poles.