Chapter 10 Jets and Outflows

The ejection of prodigious amounts of material during birth still puzzles us. It is paradoxical that just when we expect to observe infall, the signatures of outflow prevail. Ultimately, we want to know why. Are we witnessing simply the disposal of waste material – the placenta being cast aside? Or are outflows essential, a means of removing some ingredient which allows the rest to flow inwards? There are reasons to believe it is the latter, in which case outflows hold the key to the formation of individual stars. Outflows may also determine the destiny of entire clusters of stars. They remove gravitational energy from a small scale and feed it back into the large-scale cloud. Done at high speed, the cloud is disrupted and dispersed. The outflow feedback thus influences the star formation efficiency. The young cluster stars are then subject to less gravitational pull and, consequently, also disperse into looser associations and, eventually, the general field. All protostars and all young stars with accretion disks appear to drive powerful outflows of some kind. Outflows are driven by accretion. If we can understand how the outflow evolves as the accretion proceeds, we can perhaps understand how the star evolves. First, we need to investigate from where this material originates and the launching mechanism in operation. Progress has so far proven slow because the structure of each outflow is different: the determining factors are as much environmental as genetic. A variety of spectacular displays have been uncovered over the last twenty years and the common factors and causes are still not established. Outflows were at first thought to be of no great consequence to star formation and of restricted influence on their environment. They were thought to be merely outbursts lasting ten thousand years during the million-year evolution. This changed with the discovery of gigantic old outflows and the