Carbon chemistry in Galactic Bulge Planetary Nebulae
  Galactic Bulge Planetary Nebulae show evidence of mixed chemistry with
emission from both silicate dust and PAHs. This mixed chemistry is unlikely to
be related to carbon dredge up, as third dredge-up is not expected to occur in
the low mass Bulge stars. We show that the phenomenon is widespread, and is
seen in 30 nebulae out of 40 of our sample, selected on the basis of their
infrared flux. HST images and UVES spectra show that the mixed chemistry is not
related to the presence of emission-line stars, as it is in the Galactic disk
population. We also rule out interaction with the ISM as origin of the PAHs.
Instead, a strong correlation is found with morphology, and the presence of a
dense torus. A chemical model is presented which shows that hydrocarbon chains
can form within oxygen-rich gas through gas-phase chemical reactions. The model
predicts two layers, one at $A_V\sim 1.5$ where small hydrocarbons form from
reactions with C$^+$, and one at $A_V\sim 4$, where larger chains (and by
implication, PAHs) form from reactions with neutral, atomic carbon. These
reactions take place in a mini-PDR. We conclude that the mixed chemistry
phenomenon occurring in the Galactic Bulge Planetary Nebulae is best explained
through hydrocarbon chemistry in an UV-irradiated, dense torus.
