The Orion Bar represents a dramatic interface between the Orion Nebula formed by the hot young stars of the Trapezium and the cool molecular cloud from which they formed. The image above of the central regions of the Orion Nebula is from the Hubble Space Telescope. The white box shows the position of the near-infrared image (below) centered on the brightest infrared emission from the Orion Bar.
The Bar is called a photo-dissociation region (PDR), because it is here that the ionization from the Trapezium is eating its way into the parent cloud, photo-dissociating molecules into atoms and then ionizing them. We are looking at the Bar edge-on, so all of the individual layers that make up the complicated structure of a PDR are easily distinguished. Starting with the ionized region and moving to the southeast (down and left), we pass from the ionized medium into a dusty interface (visible in the HST image). Behind this, emission from warm dust rises to a maximum in the near-infrared (below).
We obtained this image at the NASA 1.5 m reflector at Mt. Lemmon (maintained by Steward Obs.), through a narrow-band filter centered at 3.29 µm, a wavelength of very strong emission from polycyclic aromatic hydrocarbons. The PAHs are excited by ultraviolet radiation which tapers off quickly into the molecular cloud, so the PAHs emit most strongly only near the front edge of the molecular cloud.
As the UV radiation continues on into the molecular cloud behind the layer of peak PAH emission, its strength continues to drop, especially at higher energies. First H can associate to form H2 molecules, and further behind, we see CO molecules. In the image below (from Tielens et al. 1993), which covers roughly the same image as our 3.3 µm image above, the PAH emission layer is coded in blue, the H2 emission in green, and the CO emission in red. The yellow area results from an overlap of green and red.
We published the results of our study on this wonderful laboratory in one refereed paper and a series of conference proceedings. See Sloan et al. (1997) for more.
Tielens, A.G.G.M., Meixner, M.M., van der Werf, P.P., Bregman, J., Tauber, J.A., Stutzki, J., & Rank, D. 1993, Science, 262, 86.
Last modified 27 May, 2008. Copyright Gregory C. Sloan.
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