Due to their bright, highly ionized emission and copious ultraviolet radiation fields, planetary nebulae have always been considered to be the realm of atomic gas. Recent observations, however, have shown that such nebulae are rich in molecular material, even for the oldest objects. For example, the Helix Nebula has widespread HCO+, HCN, HNC, CN, H2CO, CCH, and c-C3H2 emission which samples the whole of the optical image. Other nebulae such as K4-47, K3-58, K3-17, M3-28, and M4-14 contain many of these molecules as well. The molecular concentrations are significant, with fractioanl abundances as high as 10-8-10-7 relative to H2. Moreover, the abundances do not decrease with the age of the nebulae, which indicates some effective self-shielding process and/or continuous production mechanism for the molecular material. The presence of C60 in some of these nebulae further complicates the chemical scenario. Many of the molecule-rich objects of this type also have a bipolar geometry, including the so-called Helix Nebula. It is unclear whether dynamics is influencing the chemistry in these sources. Currently there are no chemical models that can explain the high molecular abundancdes in planetary nebulae, or the presence of polyatomic species. This talk will summarize the current understanding of the chemistry in these objects and possible chemical schemes that can explain the unexpected observations.

The unusual dynamics and chemistry in planetary nebulae

255th National Meeting (2018)

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