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

TL;DR

This study reveals the presence of PAHs and molecular hydrogen in the oxygen-rich planetary nebula NGC6720, suggesting in-situ formation of complex carbon molecules in such environments, challenging traditional dust chemistry classifications.

Contribution

It provides the first spatially-resolved evidence of PAHs in an oxygen-rich planetary nebula, indicating possible in-situ formation of large carbon molecules in these conditions.

Findings

PAH emission detected in NGC6720's ring

PAHs spatially correlate with molecular hydrogen emission

In-situ formation of PAHs in oxygen-rich environments suggested

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 NGC6720 reveals the presence of the 11.3 micron aromatic (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 distribution of PAHs is found to closely follow that of the warm clumpy molecular hydrogen emission. Emission from both neutral PAHs and warm H2 is 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.