Hydrocarbon Chemistry in Planetary Nebulae: Observations of CCH and c-C3H2

In an effort to fully evaluate the molecular content of planetary nebulae (PNe), and the role of hydrocarbons, we have undertaken a search for CCH towards K4-47, K3-58, K3-17, M3-28, M4-14, Hb 5, K3-45, M1-7, M3-55, NGC 2440, NGC 6772, M1-12, and M1-20. These nebulae span a range of kinematic ages and morphologies. In addition, we observed CN, CCH, and c-C3H2 at eight positions sampling the Helix Nebula. Measurements at 3 mm of the N=1→0 transitions of CCH and CN and the J=21,2→10,1 of c-C3H2 were performed using the ALMA prototype 12-M antenna of the Arizona Radio Observatory (ARO), while the N=3→2 transition of CCH at 1 mm was observed using the ARO Sub-Millimeter Telescope. CCH was detected in 9 of the 13 survey PNe, while CCH, CN, and c-C3H2 were observed at all positions in the Helix, often with a complex velocity structure; c-C3H2 was also identified in K4-47. From radiative transfer modeling, column densities for CCH were found to range between Ntot(CCH) ~ 0.2-3.3 × 1015 cm-2 for the survey PNe, corresponding to fractional abundances with respect to H2 of f(CCH) ~ 0.2-47 × 10-7. The CN, CCH, and c-C3H2 column densities across the Helix were estimated to range between Ntot(CN) ~ 6.9-74 × 1011 cm-2, Ntot(CCH) ~ 3.2-28 × 1011 cm-2, and Ntot(c-C3H2) ~ 0.2-4.7 × 1011 cm-2, with fractional abundances of f(CN) ~ 0.9-9.8 × 10-7, f(CCH) ~ 0.4-3.7 × 10-7, and f(c-C3H2) ~ 0.3-6.5 × 10-8. Based on HCN measurements of the Helix by Schmidt & Ziurys (2017a), [CN]/[HCN] ratios are ~1-34, while [CCH]/[c-C3H2] ratios varied between ~3-46. The abundance of CCH in all observed PNe did not vary significantly across the nebular lifespan of ~10,000 years, in contrast to model predictions. These abundances are ~1-2 orders of magnitude greater than those measured in the diffuse ISM; moreover, the [CN]/[HCN] and [CCH]/[c-C3H2] ratios observed in the Helix are comparable to those in diffuse clouds, striking evidence that molecular material ejected from PNe seeds the surrounding ISM. These results also suggest that the presence of CCH and c-C3H2 in PNe may be tied to the photolysis of hydrogenated amorphous carbons and the formation and/or destruction of C60.