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 \mu m 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 \mu m band , or the existence of adequate IRAS Low Resolution Spectrometer data for the 11.3 \mu m 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- \mu m and 11.3- \mu m 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 7.7- \mu m band is found to be 0.6 ^ { 0.2 } _ { 0.4 } , in good accord with that found from sensitive ground-based measurements of the 3.3- \mu band too .