Summary

The Orion Nebula is a cavity in the side of a molecular cloud, excavated by UV radiation from the Trapezium stars. Within the molecular cloud are a number of young stellar objects, one of which is responsible for a large molecular hydrogen outflow. This outflow is unusual in that it has taken the form of an array of linear structures, termed ``fingers'', radiating from a point a few arcseconds south of BN. The fingers themselves are not difficult to explain; structures of this type are often seen in outflows from YSOs. The unusual aspect is the fact that there are over 50 fingers, in a fan-shaped outflow, extending over more than 2 arcminutes (0.3 pc).

The deeply-embedded young stellar object IRc2 is generally considered the most likely candidate for the source of this outflow, but this is still undetermined. Also undetermined is the mechanism by which this array of objects was produced; one plausible model involves two winds of different densities colliding to produce Rayleigh-Taylor instabilities. These in turn form knots in situ, already accelerating, in the flow, which become the tips of the fingers. The uniqueness of this object is also due to its (probably) short lifetime; with dynamical timescales of order 1000 years, and cooling times for the shocks of  1 year, this is likely to be a very brief stage in the life of a high-mass YSO. Even if it is a common stage, its short duration means that we are fortunate to be able to observe it.

Figure 1: A diagram showing the various layers of the nebula. Not to scale.

Figure 2: HST WFPC2 image of the Orion Nebula. Red is [NII], H is green; [OIII] is blue. The box defines the region seen in Figure 3, and is about 90 arcseconds on a side. Image courtesy C.R. O'Dell.

Figure: HST NICMOS images of the BN-KL region. (a) 1.87 m Pa emission from the surface of the nebula. (b) 2.12 2 emission from the YSO outflow embedded within the molecular cloud beneath the nebula. The two fields are identical. They are about 90 arcseconds on a side, centered on BN.

Figure 4: HST NICMOS 2.15 m continuum image of the BN-KL region. This is the same field as in Figure 3. See the text for an explanation of the labels.

Figure 5: Grayscale: the bottom two-thirds of Figure 3b. The plots are velocity profiles taken from UKIRT Fabry-Perot data. Each plot is given a number, which is the number of the object in Chrysostomou et al (1997). The y-axes of the plots are arbitrary; the x-axes all range from -140 km/s to +150 km/s.

© Copyright Astronomical Society of Australia 1997