Molecular survival in evolved planetary nebulae: Detection of H ₂CO, c-C₃H₂, and C₂H in the helix

H₂CO, c-C₃H₂, and C₂H have been identified in the neutral envelope of the highly evolved planetary nebula (PN), the Helix (also know as NGC 7293). Emission from these species were detected toward a peak position in CO, 372″ east of the central star, using the facilities of the Arizona Radio Observatory (ARO). C₂H and c-C ₃H₂ were identified on the basis of their 3 mm transitions, measured with the ARO 12 m, while five lines of H₂CO were observed using the 12 m at 2 and 3 mm and the ARO Submillimeter Telescope at 1 mm. From a radiative transfer analysis of the formaldehyde emission, the molecular material was determined to have a density of n(H₂) ∼3 × 10⁵ cm^-3, with a kinetic temperature of T _kin ∼20 K. Column densities for C₂H, H₂CO, and c-C₃H₂ of N_tot ∼1.4 × 10 ¹³ cm^-2, 1.1 × 10¹² cm^-2, and 3 × 10¹¹ cm^-2, respectively, were derived, corresponding to fractional abundances relative to H₂ of f (H ₂CO) = 1 × 10^-7, f (c-C₃H₂) = 3 × 10^-8, and f (C₂H) = 1 × 10^-6 _. The physical conditions found support the notion that molecules in evolved PNe survive in dense clumps in pressure equilibrium, shielded from photodissociation. The presence of H₂CO, c-C₃H ₂, and C₂H, along with the previously observed species CN, HNC, HCN, and HCO⁺, indicates that a relatively complex chemistry can occur in the late stages of PN evolution, despite potentially destructive ultraviolet radiation. These molecules have also been observed in diffuse clouds, suggesting a possible connection between molecular material in evolved PNe and the diffuse ISM.