The Molecular Content of Planetary Nebulae: The Next Level

The planetary nebula (PN) stage is the final phase for the evolution of most stars. It is preceded by the asymptotic giant branch (AGB), in which mass loss creates a circumstellar envelope. These envelopes are well-known to have a rich, gas-phase chemistry. As a star leaves the AGB, mass loss is accelerated, leaving behind a hot, ultraviolet-emitting white dwarf that ionizes the surrounding remnant envelope. Chemical models have predicted that the intense UV flux in the PN stage should photodissociate most molecules created in the AGB phase. However, recent detections of a variety of polyatomic species, including HCN, HCO⁺, CCH, HNC, and c-C₃H₂ in approximately 20 PNe, have shown that the modeling is inaccurate. Not only are these molecules relatively prevalent in these objects, but their abundances remain almost constant over the anticipated PN lifespan of ~10,000 years. Some of these objects are well-known, such as the Helix, Butterfly, and Ring Nebulae. Very recently, observations of the J = 1 → 0 and the J = 3 → 2 transitions of HCN and HCO⁺ have been undertaken in an additional 13 PNe, using the Submillimeter Telescope (SMT) and the 12 m antenna of the Arizona Radio Observatory (ARO). These objects include Hu1-1, M1-8, M1-59, and M4-17. Detections have been made in 10 nebulae for HCN and 11 nebulae for HCO⁺. The success of this search, combined with the results of past molecular surveys of PNe, demonstrates that molecules survive in significant quantities in PNe and therefore are seeding the surrounding interstellar medium. These new results will be presented, including radiative transfer analysis of the detected transitions and abundance estimates. Implications for the important role that PNe play in the cosmic recycling of molecular material will be discussed.