2013ApJ...765..112Z

Observations of CO, HCO⁺, and H₂ CO 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 H₂ CO (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_⟶20, 2_) were conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO⁺ and H₂ CO 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 x10⁵/cm3. 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∼ 0.25-4.5x10¹⁵/cm2, with a fractional abundance of f (CO/H₂) ∼ 0.3-6x10^–4. Column densities for HCO⁺ and H₂ CO were determined to be N_tot∼ 0.2-5.5x10¹¹/cm2 and 0.2-1.6x10¹²/cm2, respectively, with fractional abundances of f (HCO⁺/H₂) ∼ 0.3-7.3x10^–8 and f (H₂CO/H₂) ∼ 0.3-2.1x10^–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.