G. C. Sloan, Jesse Bregman, A. S. B. Schultz (NASA Ames)
P. Temi, D. M. Rank (UCO and Lick Observatory)
1996, in The Role of Dust in the Formation of Stars, ed. H.U. Käufl & R. Siebenmorgen, (Springer Verlag), 63
NOTE: in the published version of this paper, Fig. 3 is incorrect; the correct version is (or will be) presented here.
Polycyclic aromatic hydrocarbons (PAHs) have been proposed as the carrier of the well-known series of spectral features at 3.3, 3.4, 6.2, 7.7, 8.6, 11.3, and 12.7 µm. Here, we concentrate on the PAH features in the 3 µm regime: the main band at 3.29 µm, which arises from a C-H stretch (v=1-0) on the periphery of an aromatic ring, and the satellite band at 3.4 µm. Recent spectra by Geballe et al. (1994), Joblin et al. (1996), and Sloan et al. (1996) suggest that the satellite band is a blend of (1) a sidegroup band at 3.40 µm arising from a C-H stretch on an aliphatic (CH3) molecule attached to a PAH molecule and (2) a hot band at 3.43 µm arising from an excited (v=2-1) counterpart of the 3.29 µm band. The sidegroup band appears to dominate the hot band in most cases.
Photodissociation regions (PDRs) lie at the interface between H II regions and molecular clouds and usually exhibit strong emission in the PAH bands. In both M17 Southwest and the Orion Bar, we can observe PDRs with a favorable edge-on geometry. Our goal is to use narrow band imaging of the PAH bands at 3.3 and 3.4 µm to probe the physics of both of these PDRs. In particular, we will compare our results to the extensive models of the Orion Bar.
We use narrow-band imaging at three wavelengths to isolate the main band (3.29 µm), the PAH pedestal (3.36 µm), and the satellite band (3.42 µm). We analyze the band strengths F3.29 and F3.42 and the ratio F3.42/F3.29 after subtracting the pedestal emission. We observed the Orion Bar in 1995 February and M17 SW in 1995 May at the NASA 1.5 m telescope at Mt. Lemmon/Steward Obs. with a LN2 cooled Amber Engineering 128x128 InSb array. The pixels span 0.78" each. A circularly variable filter wheel (CVF) isolated the bandpasses.
Models of the emission from CO and H2 in the Bar by Tielens et al. (1993) show that the UV radiation drops exponentially into the Bar: FUV ~ exp(-d/d0), with a 1/e folding distance d0 = 3". Observations of the PAH emission at 3.3 µm by Giard et al. (1994) result in a very different value of the folding distance: d0 = 9". They suggest that clumping within the Bar may account for this discrepancy.
Narrow-band images of the Orion Bar at 3.29 µm (top) and 3.42 µm (bottom). The box defines the region used to determine the flux profiles and ratios in Fig. 3. The three circles depict the approximate beam positions of the spectra of Geballe et al. (1989). The star to the left is theta2 Ori A.
Narrow-band images of M17 Southwest at 3.29 µm (left) and 3.42 µm (right). The rectangle surrounds the area used to generate the profiles in Fig.3.
Flux ratios and profiles for the Orion Bar and M17 SW, plotted as a function of distance from the front of the PDR. Top: A comparison of the 3.29 µm flux (filled circles) and 3.42 µm flux (open diamonds) from the Orion Bar with exponential drop-offs (dashed lines). The ratio F3.42/F3.29 in the Orion Bar is plotted in the middle panel, along with ratios measured spectroscopically by Geballe et al. (1989) in three discrete locations (heavy filled circles). Bottom: The 3.29 µm flux (filled circles) and 3.42 µm flux (open diamonds) in M17 SW plotted with an exponential drop-off (dashed line).
Our 3.29 and 3.42 µm images show significant differences in the flux distribution in these two bands along the Bar, suggesting that the PAH material in the Bar has more than one component. We determine that the 1/e folding distance (d0) is 9" at 3.29 and 3.42 µm (confirming Giard et al. 1994). The discrepancy with d0 derived from molecular emission suggests that the molecular emission and PAH emission are not tracing the same UV field. The flux ratio F3.42/F3.29 generally increases with distance from the front of the Bar. This was first seen in the multi-aperture spectra taken by Geballe et al. (1989), but our data extend further into the molecular region and show additional structure.
A clumpy composition in the Bar could account for the discrepancy between the UV extinction as measured from PAH features and molecular emission (Giard et al. 1994); it also produces better models of sub-mm and radio observations (Tauber et al. 1994; Hogerheijde et al. 1995). Tauber et al. (1994) suggest that the interclump region has a density of ~5×104 cm-3 and a filling factor of ~1-8%. The corresponding values as determined by Hogerheijde et al. (1995) are ~3×104 cm-3 and ~0.5%. The clumps would have densities of order ~106-7 cm-3.
Our results suggest that F3.42/F3.29 would be ~0.12 between the clumps, but could be as high as 0.7 within the clumps. This high ratio within the clumps makes it very unlikely that the hot band could produce the observed 3.42 µm emission; most of it must come from aliphatic sidegroups.
We suggest that the PAHs within the clumps are primitive and unprocessed, while the harsh UV field in the interclump region has stripped the PAHs there of their aliphatic sidegroups.
Our images show that d0 = 5", both for F3.29 and F3.42. Since the density is proportional to the linear folding distance and M17 SW is 4.4 times further away than the Orion Bar (2.2 kpc compared to 0.5 kpc), it follows that the Bar is 2.4 times as dense as M17 SW.
Two distinct PAH populations exist within PDRs. Primitive, unprocessed PAHs with many attached aliphatic sidegroups can survive within clumps which protect them from harsh UV radiation. In the interclump region, the radiation field has processed the PAHs, stripping most of their aliphatic sidegroups away.
The molecular emission and PAH emission do not trace the same regions within PDRs. Molecules trace the optically thickest lines of sight to the ionization front, while the PAHs trace optically thinner lines of sight. Also, the PAHs only trace conditions close to the ionization front, in regions of strong UV intensity, while more fragile molecular species like CO and H2 trace regions with reduced UV radiation.
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Last modified 27 May, 2008. © Gregory C. Sloan and others.
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