title: Polycyclic aromatic hydrocarbons and molecular hydrogen in oxygen-rich planetary nebulae: the case of NGC 6720.
abstract: Evolved stars are primary sources for the formation of polycyclic aromatic hydrocarbons (PAHs) and dust grains. Their circumstellar chemistry is usually designated as either oxygen-rich or carbon-rich, although dual-dust chemistry objects, whose infrared spectra reveal both silicate- and carbon-dust features, are also known. The exact origin and nature of this dual-dust chemistry is not yet understood.Spitzer-IRS mid-infrared spectroscopic imaging of the nearby, oxygen-rich planetary nebula NGC 6720 reveals the presence of the 11.3μm aromatic (PAH) emission band. It is attributed to emission from neutral PAHs, since no band is observed in the 7-8μm range. The spatial distribution of PAHs is found to closely follow that of the warm clumpy molecular hydrogen emission. Emission from both neutral PAHs and warm H2is likely to arise from photo-dissociation regions associated with dense knots that are located within the main ring. The presence of PAHs together with the previously derived high abundance of free carbon (relative to CO) suggest that the local conditions in an oxygen-rich environment can also become conducive to in-situ formation of large carbonaceous molecules, such as PAHs, via a bottom-up chemical pathway. In this scenario, the same stellar source can enrich the interstellar medium with both oxygen-rich dust and large carbonaceous molecules.
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, 31028, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse, France.
authors.forename: N L J
authors.initials: NL
authors.lastname: Cox
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, 31028, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse, France.
authors.forename: P
authors.initials: P
authors.lastname: Pilleri
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, 31028, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse, France.
authors.forename: O
authors.initials: O
authors.lastname: Berné
authors.affiliation: Group of Molecular Astrophysics, ICMM, CSIC, C/Sor Juana Inés de La Cruz N3, E-28049, Madrid, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Cernicharo
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, 31028, Toulouse, France ; CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse, France.
authors.forename: C
authors.initials: C
authors.lastname: Joblin
keywords: H ii regions; NGC 6720; dust; individual; infrared; molecules; planetary nebulae; stars