Production of the Fingers

Any of the fingers standing alone would be fairly unremarkable. Young stars frequently possess outflows which, upon encountering ambient material, result in shocks set up in both the ambient and outflow material. If the outflowing material takes the form of fast-moving knots, or of a jet, the resultant shocks can take the form seen in the fingers in Orion. (Few of these, however, display the intricate structure seen in the inner fingers, which may be instrinsic to the finger, or may be the result of a superposition of individual fingers in our line of sight.)

What makes this nebula a remarkable object is the large finger array. The first explanation that may come to mind is that it is produced by a series of jets, or a precessing jet. A precessing jet would have to be intermittent, and of short duration, to produce the distinct individual fingers seen. While a single object may have two oppositely-directed jets, scores of jets would be needed to produce the array, and it is difficult to imagine a mechanism that by which so many individual YSOs might produce jets with similar velocities, aligned in such a way, within a relatively short period of time.

Current theories posit a simultaneous, common source for the fingers. Perhaps the most intuitive explanation is that the fingers are the result of a large number of clumps accelerated by some explosive event within the cloud. However, theoretical studies of clump acceleration (e.g. Stone & Norman 1992; Xu & Stone 1995) have found that clumps are fragmented, rather than accelerated, producing a very different morphology. Another model is that of Stone, Xu, & Mundy (1995); in this scenario the knots are formed and accelerated in situ through Rayleigh-Taylor instabilities. These result from the mixing of winds of different densities, such as the collision of an outflow with a previous, less dense outflow. The two separate winds might be the result of successive outflows from a single source, or possibly the second outflow could result from a separate, but nearby source. For a more detailed examination of this model and its relevance to Orion KL, see Burton & Stone 2001.

© Copyright Astronomical Society of Australia 1997