“PAH” emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio

Airborne and space-based low-resolution spectroscopy in the 1980s discovered tantalizing quantitative relationships between the gas phase C/O abundance ratio in planetary nebulae (PNe) and the fractions of total far-infrared luminosity radiated by the 7.7 and 11.3 μm bands (the C=C stretch and C-H bend, respectively), of polycyclic aromatic hydrocarbons. Only a very small sample of nebulae was studied in this context, limited by airborne observations of the 7.7 μm band, or the existence of adequate IRAS Low Resolution Spectrometer data for the 11.3 μm band. To investigate these trends further, we have expanded the sample of planetaries available for this study using Infrared Space Observatory (ISO) low-resolution spectra secured with the Short Wavelength Spectrometer (SWS) and the Long Wavelength Spectrometer (LWS). The new sample of 43 PNe, of which 17 are detected in PAH emission, addresses the range from C/O = 0.2−13 with the objective of trying to delineate the pathways by which carbon dust grains might have formed in planetaries. For the 7.7-μm and 11.3-μm bands, we confirm that the ratio of band strength to total infrared luminosity is correlated with the nebular C/O ratio. Expressed in equivalent width terms, the cut-on C/O ratio for the 7.7μm band is found to be 0.6 0.4 , in good accord with that found from sensitive ground-based measurements of the 3.3-μ band too.