Probing Irradiated Molecular Gas in the Planetary Nebulae NGC 7027 and NGC 7293 (the Helix)

Planetary nebulae (PNe) represent the near endpoints of evolution for stars of initial mass ~1-8 M⊙, wherein the envelope of an asymptotic giant branch (AGB) star is photodissociated and ionized by high-energy radiation from a newly emerging white dwarf that was the progenitor star's core. Though best known as ~104 K optical emission line sources, a subset of PNe retain significant masses of cold, dense molecular gas that is irradiated from within by UV from PN central stars (CSPNe) --- and, in certain nebulae, by X-rays from the CSPN or wind-collision-generated shocks. This makes PNe fertile ground for studies of radiation-driven molecular gas heating and chemistry, with the results potentially applicable to a diverse range of astrophysical environments. We report on IRAM/NOEMA radio interferometric observations of the well-studied, young PN NGC 7027 as well as an IRAM 30 m spectral line survey of selected regions within the molecule-rich, globule-studded Helix Nebula (NGC 7293). Our NOEMA CO+ map of NGC 7027 is among only a handful obtained for any astrophysical object --- and the first map of a planetary nebula --- obtained in this key tracer of molecular ionization. Although X-ray-driven chemistry is typically invoked to explain CO+ abundance enhancements in molecular gas, preliminary analysis indicates that UV may dominate the production of CO+ in NGC 7027. Our 30 m spectra of the Helix Nebula have yielded first-time detections of numerous species, including HCN, HNC, and HCO+, within individual globules. We use these data to investigate the utility of the HNC/HCN abundance ratio as diagnostic of UV-driven heating of molecular gas.