Jesse Bregman & G.C. Sloan (NASA Ames Research Center)
1996, in From Stardust to Planetesimals: Contributed Papers, ed. M.E. Kress, A.G.G.M. Tielens, & Y.J. Pendleton, NASA-CP 3343, 121 (Moffett Field, CA: NASA)
We have investigated the emission from polycyclic aromatic hydrocarbons (PAHs) in the Orion Bar region using a combination of narrow-band imaging and long-slit spectroscopy. Our goal is to study how the strength of the PAH bands vary with spatial position in this edge-on photo-dissociation region. Our specific focus here is how these variations constrain the carrier of the 3.4 µm band. The 3.29 µm band arises from a v = 1 — 0 aromatic C-H stretch. The 3.40 µm band may arise from a v = 2 — 1 aromatic C-H stretch (Barker et al. 1987), or an aliphatic C-H stretch in either attached methyl sidegroups (Joblin et al. 1996) or PAHs with more than one H atom attached to each C atom (H-PAHs; Bernstein et al. 1996a, 1996b).
Sloan et al. (1995) obtained narrow-band images of the Bar at 3.3 and 3.4 µm (Fig. 1) in order to compare the behavior of the 3.29 and 3.40 µm bands with distance from the ionization front. In a rectangle ~16" across, both bands peak at the ionization front and then decrease exponentially into the neutral region. The 3.29 µm band has a 1/e scale height ~9" but the 3.40 µm band shows an excess ~10" behind the front. The positions of the H2 and CO emission layers at 10" and 20" imply that the UV field decreases with a 1/e scale height of 3" (Tielens et al. 1993), a discrepancy which Giard et al. (1994) resolve by suggesting that the Bar has a very clumpy distribution. The PAH emission would occur in the regions between the clumps, and the longer attenuation distances would reflect the lower densities in these regions. The molecular emission would arise from shielded regions in and behind clumps where they are protected from the UV field. The higher opacities in the clumps would lead to different scale heights than in the interclump region.
Contour plots of the 3.29 and 3.42 µm narrow-band images. The narrow rectangle in the 3.29 µm image depicts the position of the slit used for the spectroscopy. The five rectangles in the 3.40 µm image show the regions used to generate the profiles in Fig. 2.
Two PAH populations would explain the differences in the spatial behaviors of the 3.29 and 3.40 µm bands. Primitive unprocessed PAHs reside in and behind the clumps, while the PAHs in the interclump region have been processed by the UV field. In simple geometric models of these two distributions, in the primitive PAHs, the ratio of emission in the 3.40 µm band to the 3.29 µm band exceeds by more than a factor of two the limit of 0.17 for a v = 2 — 1 transition set by Schutte et al. (1993). The energies required to produce a flux ratio higher than this limit would disrupt the C-H bond.
Sloan et al. (1996) obtained long-slit spectroscopy with the slit oriented perpendicular to the Bar, and showed that the 3.40 µm feature consists of two components. The main component (~3.395 µm) has a distribution similar to the 3.29 µm band, but the extra component (~3.405 µm) has a distribution very similar to the H2 emission, peaking ~10" behind the ionization front. They speculate that the extra component might arise from attached methyl sidegroups while the main component arises from H-PAHs. Within the two-component PAH model, the methyl sidegroups would belong to the primitive PAHs, and the H-PAHs would be part of the processed PAHs.
We have modified our previous analysis of the narrow-band images of the Orion Bar by subdividing the Bar into five strips perpendicular to the ionization front, each five arcseconds across (Fig. 2). All of the strips show similar behaviors, but there are differences, primarily in the relative positions of the peaks of the 3.29 µm and 3.40 µm profiles, and also in secondary structures ~ 10" behind the ionization front.
Profiles of the 3.29 and 3.40 µm emission features, extracted from the images in Fig. 1. The top panel corresponds to the east-most (or left-most) strip in Fig. 1; the bottom panel is the west-most profile. In each panel, the 3.29 µm profile is a solid line and the 3.40 µm profile is a dashed line. For clarity, the 3.40 µm profiles are plotted at ten times the scale of the 3.29 µm profiles.
Our narrow-band imaging shows that the behavior of the 3.29 and 3.40 µm bands with distance from the ionization front varies along the Orion Bar. These differences point to the need to obtain additional long-slit spectroscopy of the Orion Bar with the slit in the perpendicular orientation. Such data would (1) test whether the results of Sloan et al. (1996) can be generalized along the Bar, and (2) exploit the inhomogeneities in the Bar to further disentangle the various spectral emission components.
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Last modified 5 June, 2008. © Gregory C. Sloan and others.
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