MOLECULAR SURVIVAL IN EVOLVED PLANETARY NEBULAE: DETECTION OF H 2 CO, c-C 3 H 2 , AND C 2 H IN THE HELIX

Abstract

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-C3H2 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 × 1013 cm–2, 1.1 × 1012 cm–2, and 3 × 1011 cm–2, respectively, were derived, corresponding to fractional abundances relative to H2 of f (H2CO) = 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-C3H2, 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.