Chemical Complexity in the Helix Nebula: Multi-line Observations of H2CO, 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 = 10, 1 → 00, 0, 21, 2 → 11, 1, 20, 2 → 10, 1, 21, 1 → 11, 0, and 30, 3 →20, 2) were conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO+ and H2CO 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(H2) ~ 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.