  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 micron 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 micron band, or the existence of adequate IRAS Low Resolution
Spectrometer data for the 11.3 micron 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-micron and 11.3- micron 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-micron
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-micron band too.
