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.
