Chemical complexity in the helix Nebula: Multi-line observations of H ₂CO, HCO⁺, and CO

Observations of CO, HCO⁺, and H₂CO have been carried out at nine positions across the Helix Nebula (NGC 7293) using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. Measurements of the J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, two transitions of HCO⁺ (J = 1 → 0 and 3 →2), and five lines of H₂CO (J _{Ka, Kc} = 1_{0, 1} → 0 _{0, 0}, 2_{1, 2} → 1_{1, 1}, 2_{0, 2} → 1_{0, 1}, 2_{1, 1} → 1_{1, 0}, and 3 _{0, 3} →2_{0, 2}) were conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO⁺ and H ₂CO were detected at all positions, along with three transitions of CO. From a radiative transfer analysis, the kinetic temperature was found to be T _K ∼ 15-40 K across the Helix with a gas density of n(H ₂) ∼ 0.1-5 × 10⁵ cm^-3. The warmer gas appears to be closer to the central star, but high density material is distributed throughout the nebula. For CO, the column density was found to be N _tot ∼ 0.25-4.5 × 10¹⁵ cm^-2, with a fractional abundance of f (CO/H₂) ∼ 0.3-6 × 10 ^-4. Column densities for HCO⁺ and H₂CO were determined to be N _tot ∼ 0.2-5.5 × 10¹¹ cm ^-2 and 0.2-1.6 × 10¹² cm^-2, respectively, with fractional abundances of f (HCO⁺/H₂) ∼ 0.3-7.3 × 10^-8 and f (H₂CO/H₂) ∼ 0.3-2.1 × 10^-7 - several orders of magnitude higher than predicted by chemical models. Polyatomic molecules in the Helix appear to be well-protected from photodissociation and may actually seed the diffuse interstellar medium.