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Evolved stars are primary sources for the formation of polycyclic aromatichydrocarbons (PAHs) and dust grains. Their circumstellar chemistry is usuallydesignated as either oxygen-rich or carbon-rich, although dual-dust chemistryobjects, whose infrared spectra reveal both silicate- and carbon-dust features,are also known. The exact origin and nature of this dual-dust chemistry is notyet understood. Spitzer-IRS mid-infrared spectroscopic imaging of the nearby,oxygen-rich planetary nebula NGC6720 reveals the presence of the 11.3 micronaromatic (PAH) emission band. It is attributed to emission from neutral PAHs,since no band is observed in the 7 to 8 micron range. The spatial distributionof PAHs is found to closely follow that of the warm clumpy molecular hydrogenemission. Emission from both neutral PAHs and warm H2 is likely to arise fromphoto-dissociation regions associated with dense knots that are located withinthe main ring. The presence of PAHs together with the previously derived highabundance of free carbon (relative to CO) suggest that the local conditions inan oxygen-rich environment can also become conducive to in-situ formation oflarge carbonaceous molecules, such as PAHs, via a bottom-up chemical pathway.In this scenario, the same stellar source can enrich the interstellar mediumwith both oxygen-rich dust and large carbonaceous molecules.

Polycyclic aromatic hydrocarbons and molecular hydrogen in oxygen-rich planetary nebulae: the case of NGC6720