Airborne observations of the infrared emission bands

Low resolution airborne spectra from 5 to 8 μm are now available for a sample of 40 sources selected from the IRAS LRS Atlas with PAH emission features. A new emission band at 5.2 μm, previously predicted for PAHs, has been found in 33 sources; it also correlates with the 7.7 μm band. This extends the generic spectrum of narrow observed PAH features to 3.3, 5.2, 5.6, 6.2, 6.9, 7.7, 8.7, 11.3, and 12.7 μm. From this sizeable sample of sources we define the relative strengths of most of these bands in three separate nebular environments: planetaries, HII regions, and reflection nebulae. There are significant differences in the generic spectra of PAHs in these different environments, particularly in the ratio of I (6.2)/I (7.7) and in the actual wavelength of the strong 7.7 μm peak. New observations of southern nebulae, including the unusually carbon-rich [WC10] planetary nuclei and the apparently oxygen-rich object, NGC 6302, broadens the range of C/O over which we detect PAH features to 0.2-4.8. The addition of these planetaries strengthens our earlier claim that the fraction of total emission observed by IRAS that is carried by PAH emission is correlated with nebular gas-phase C/O ratio. The [WC10] nuclei also exhibit a strong plateau of emission linking the 6.2 and 7.7 μm features