HCN and HCO+ in Planetary Nebulae: The Next Level

Observations of HCN and HCO + have been carried out toward 13 planetary nebulae (PNe) using the facilities of the Arizona Radio Observatory (ARO). These nebulae represent a wide range of morphologies and ages (∼2000–28,000 yr). For both molecules, the J = 1 → 0 transitions at 88–89 GHz and the J = 3 → 2 lines at 265–267 GHz were measured, together with CO lines ( J = 1 → 0, 2 → 1, and 3 → 2, depending on the source), using the ARO 12 m and Submillimeter Telescopes. HCN and HCO + were detected with at least one transition in 10 nebulae: He 2-459, Hu 1-1, K3-52, K3-65, M1-8, M1-40, M1-59, M2-53, M4-17, and NGC 6445. HCO + was additionally identified via two transitions in Na 2. Some observed line profiles were complex, with multiple velocity components tracing varied outflows. From radiative transfer modeling, column densities were established for HCN and HCO + : N tot (HCN) = 0.005–1.1 × 10 14 and N tot (HCO + ) = 0.008–9.5 × 10 13 cm −2 . Gas densities of n (H 2 ) ∼ 10 5 –10 7 cm −3 were also determined for all PNe. Fractional abundances with respect to H 2 , calculated using CO as a proxy, are f (HCN) ∼ 0.2–1.5 × 10 −7 and f (HCO + ) ∼ 0.3–5.1 × 10 −8 . The abundances of HCN and HCO + did not significantly vary with nebular age to 28,000 yr. Combined with previous observations, at least 30 PNe contain HCN and/or HCO + , indicating that polyatomic molecules are common constituents of these objects. The data strongly support a scenario where dense ejecta from PNe seed the interstellar medium with molecular material.