Beneath the Nebula

At 2 m, the brightest object in the region is the Becklin-Neugebauer object (BN), thought to be about 0.5 pc beneath the surface of the Orion Nebula. BN is accompanied by the infrared Kleinmann-Low (KL) nebula, as well as the molecular hydrogen outflow. The entire infrared nebula is often referred to as BN-KL. Although BN dominates the 2 m image, it is not the most luminous object in the cluster. The most luminous object is IRc2, about 9 arcseconds to the southeast of BN, with a total luminosity of 2-10 x 10⁴ L (Genzel & Stutzki 1989). This deeply-embedded object has long been considered a likely candidate to be the exciting source for the outflow, both because of its luminosity and its central location within the outflow. However, Dougados et al (1993) has found that at 3.8 m IRc2 is resolved into four objects (although the polarization data of Menten & Reid (1995) suggest that not all of these objects are self-luminous). In addition, Gezari, Backman, & Werner (1998) find that the total luminosity of IRc2 is only 10³ L. Therefore, the identification of IRc2 as the source of the BN-KL outflow remains tentative.

Beckwith et al (1978), using a single-beam spectrometer, found 2 emission in a large, two-lobed structure in the BN-KL region. In 1984, Taylor et al found that the 2 actually took the form of linear structures; in the same year, Axon & Taylor discovered optical bow shocks (recognized as HH objects) up to 2 arcminutes north of the Trapezium. Allen & Burton (1993) confirmed that the 2 structures (now known as ``fingers'') and optical HH objects were related. Their images showed that the fingers were capped by bow shocks prominent in [FeII] 1.64 m emission. [FeII] is produced by shocks similar to those which emit [SII] and [OI]; in other words, the caps are the optical objects found by Axon & Taylor (1984). In most of these objects, the H₂ emission is quite separate from the [FeII], indicating that the shocks are fast enough to dissociate H₂ at the heads of the shocks. The correspondence of optical and [FeII] emission confirms that these are fast shocks. The presence of the optical emission also implies that these objects are moving into a region of lower extinction, i.e., out of the molecular cloud and into the PDR.

The outflow contains over fifty of these molecular hydrogen fingers--although counting them is difficult because they do not all have a linear morphology, and they often overlap--extending up to two arcminutes from BN/IRc2. The total mass of the bullets is  10^-3 M, and the energy in the outflow  5 x 10⁴⁶ ergs (5 x 10³⁹ J). The dynamical lifetime of the northernmost fingers, i.e. the time necessary for them to reach their current locations, is less than 1000 years (Allen & Burton 1993; Burton & Stone 2001). The proper motion studies of Lee & Burton (2000) show that these northern fingers are moving at velocities of roughly 200 km/s.

Although these northern molecular hydrogen fingers were found by ground-based observations, the inner regions of the outflow, within about 50 arcseconds of BN, still appeared as a smooth fan-shaped region. Recent HST images (Stolovy et al 1998; Schultz et al 1999) have shown that this region, too, is composed of linear, finger-like structures.

Figure 2 contains the well-known optical HST WFPC2 image of the center of the Orion Nebula (O'Dell & Wong 1996); the box delineates the area shown in Figure 3. Figure 3a shows an HST NICMOS image of this region in the light of Pa at 1.87 m, which comes from the photo-excited gas at the surface of the nebula, and is very similar in appearance to the H emission shown in Figure 2. Figure 3b shows the 2 emission from the BN-KL outflow, clearly showing the linear finger structures (both of these figures have been continuum-subtracted, and dark spots show regions where stars have been removed). This image only covers the central 90 arcseconds of the outflow; the northern fingers described above are off the field. The difference in morphology between these two wavelengths is not due to differential extinction between 1.87 and 2.12 m, but instead reveals the difference between the emission from the surface of the nebula, and the YSO outflow embedded in the molecular cloud beneath it.

In addition to the northern fingers (201, 205-207, 210), there are also a number of other objects in the region which exhibit optical and [FeII] emission (HH 208 is one of these; most of the others are as yet unnamed). This emission generally is highly blue-shifted (O'Dell et al 1997), indicating that they are Herbig-Haro objects rather than part of the nebular emission. A number of these are also associated with 2 emission (see Figure 1 of Schultz et al 1999). The fact that they are associated with optical emission implies that they are moving out of the molecular cloud and into the PDR, much like the northern fingers. Figure 1 shows a possible outflow orientation, in which some fingers are beginning to emerge from the cloud into the PDR.

© Copyright Astronomical Society of Australia 1997