Widespread CCH and c-C₃H₂ in the Helix Nebula: Unraveling the Chemical History of Hydrocarbons

The hydrocarbons CCH and c-C₃H₂ have been detected at multiple positions distributed across the Helix, along with the CN, indicating that these molecules are present throughout this very old planetary nebula. The species were identified by observations of the N = 1 → 0 transitions of CCH and CN and the J = 2_1,2 → 1_0,1 transition of c-C₃H₂, conducted with the new Atacama Large Millimeter/submillimeter Array (ALMA) prototype 12 m antenna of the Arizona Radio Observatory at 3 mm in wavelength. Column densities of the three species were calculated to be N _tot(CCH) ∼ (0.8-6.6) 10¹² cm^-2, N _tot(c-C₃H₂) ∼ (0.3-5.9) 10¹¹ cm^-2, and N _tot(CN) ∼ (0.7-7.4) 10¹² cm^-2, corresponding to fractional abundances, with respect to H₂, of f(CCH) ∼ (1.1-8.8) 10^-7, f(c-C₃H₂) ∼ (0.4-7.9) 10^-8, and f(CN) ∼ (0.9-9.9) 10^-7. Surprisingly, the abundances are comparable to those measured in younger planetary nebulae (PNe), indicating that predicted photodestruction processes for gas-phase molecules are inefficient in these objects, even over long timescales. The distributions of these two hydrocarbons in the Helix appear to be correlated, with a ratio of [CCH]/[c-C₃H₂] ∼ 5-80 and an average value of 29 6 - remarkably similar to both the range ([CCH]/[c-C₃H₂] ∼ 11-53) and average value (28.2 1.4) found in diffuse clouds. The abundances of CCH, c-C₃H₂, and CN in the Helix are also factors of 10-100 greater than those measured in diffuse gas. These results suggest that molecular ejecta from PNe are a possible external source for polyatomic species found in diffuse clouds, including C₆₀.