C/O abundance ratios and dust features in galactic planetary nebulae

The iron depletion factors found in Galactic planetary nebulae (PNe) span over two orders of magnitude, suggesting that there are differences in the grain formation and destruction processes from object to object. We explore here the relation between the iron depletions, the infrared dust features, and the C/O abundance ratios in a sample of Galactic PNe. We find that those objects with C/O<1 show a trend of increasing depletions for higher values of C/O, whereas PNe with C/O>1 break the trend and cover all the range of depletions. Most of the PNe with C/O<1 show silicate features, but several PNe with C-rich features have C/O<1, probably reflecting the uncertainties associated with the derivation of C/O. PAHs are distributed over the entire range of iron depletions and C/O values.