Carbon chemistry in Galactic bulge planetary nebulae

Galactic bulge planetary nebulae show evidence of mixed chemistry with emission from both silicate dust and polycyclic aromatic hydrocarbons (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. Hubble Space Telescope (HST) images and Ultraviolet and Visual Echelle Spectrograph (UVES) spectra show that the mixed chemistry is not related to the presence of emission-line stars, as it is in the Galactic disc population. We also rule out interaction with the interstellar medium (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~ 1.5, where small hydrocarbons form from reactions with C+, and one at A_V~ 4, where larger chains (and by implication, PAHs) form from reactions with neutral, atomic carbon. These reactions take place in a mini-photon-dominated region (PDR). We conclude that the mixed-chemistry phenomenon occurring in the Galactic bulge planetary nebulae is best explained through hydrocarbon chemistry in an ultraviolet (UV)-irradiated, dense torus.