2016ApJ...817..175S

Searches for HCN and HCO⁺ have been conducted toward 17 planetary nebulae (PNs) in the age range 800 to 13,000 years using the facilities of the Arizona Radio Observatory (ARO). For both molecules, observations of the J = 1 ⟶ 0 transition near 88-89 GHz were carried out with the ARO 12 m, including measurements with the new ALMA prototype antenna, while the J = 3 ⟶2 lines near 265-267 GHz were sought with the ARO Sub-Millimeter Telescope (SMT). HCN and HCO⁺ were newly detected in 13 of the 17 target sources in at least one transition. Nine PNs were common to both molecules: Hb5, K3-17, K3-58, M1-7, M4-14, M3-28, M3-55, NGC 2440, and K4-47, while HCO⁺ was also identified in K3-83 and M2-9, and HCN in K3-45 and NGC 6772. From radiative transfer modeling, column densities for HCN and HCO⁺ in these sources were determined to be N_tot(HCN) ∼ 0.2-27x10¹³/cm2 and N_tot ∼ 0.3-8.7x10¹³/cm2. Gas densities, assumed to be in clumped regions, were established to be n(H₂) ∼ 0.1-5.2x10⁶/cm3. Fractional abundances, relative to H₂, for both molecules were found to be f(HCN) ∼ 0.1-9.1x10^–7 and f(HCO⁺) ∼ 0.04-7.4x10^–7. The abundances of both species were found to remain relatively constant with nebular age over a 10,000 year time span, in contrast to predictions of chemical models. The HCN/HCO⁺ ratio varied from 17 to <0.2, and roughly correlates with the C/O ratio. Polyatomic molecules appear to be common constituents of PNs.