PAHs and H2 in the Ring Nebula

Nick Cox, P. Pilleri, O. Berne, J. Cernicharo & C. Joblin

IRAP/CNRS, France

We present a new analysis of Spitzer spectroscopic imaging of the Ring Nebula (NGC6720) which reveals for the first time the presence of polycyclic aromatic hydrocarbons in the main dust ring of this high-excitation oxygen-rich (C/O~0.6) Planetary Nebula. The spatial distribution of H2 and PAH emission are closely related and appear to be associated with dense knots (previously imaged at high resolution in the near-infrared H2 line) that are located inside the ionised region surrounding the hot (T~120.000 K) central star. The knots are exposed to a mild radiation field of G0 ~ 200. The infrared H2 emission corresponds to warm (T~620 K) molecular gas with similar conditions as found previously for the Helix Nebulae. We find an unusual low value for the I(7.7)/I(11.3) ratio which can be understood if all emitting PAHs are neutral. We discuss a scenario in which photo-dissociation of CO produces large quantities of free carbon which is necessary to start a rich carbon-chemistry (converting from an initially oxygen-rich gas) that provides a scheme for the bottom-up formation of large carbonaceous molecules. We address the implications these new results have for our understanding of the chemical enrichment of the interstellar medium by evolved stars.

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