Exotic Carbon Chemistry in a Planetary Nebula: The Unusual Case of K4-47

Millimeter molecular line observations have been conducted toward the young (∼900 yr) bipolar planetary nebula (PN) K4-47, using the 12 m antenna and the Submillimeter Telescope of the Arizona Radio Observatory, and the Institut de Radioastronomie Millimétrique 30 m Telescope. Measurements at 1, 2, and 3 mm of multiple transitions were carried out to ensure the accuracy of all molecular identifications. K4-47 was found to be unusually chemically rich, containing three complex species, CH₃CN, H₂CNH, and CH₃CCH, which have never before been observed in a planetary nebula. In addition, HC₃N, N₂H⁺, H₂CO, c-C₃H₂, and SiO have been identified in this object, as well as a variety of ¹³C-substituted isotopologues ({{{{H}}}₂}¹³CO, c-¹³CCCH₂, c-CC¹³CH₂, {CH}}₃}¹³CN, ¹³CH₃CN, {CH}}₃}¹³CCH, and ¹³CH₃CCH), including all three doubly¹³C-substituted varieties of HC₃N—the first known object in which all three species have been detected. After CO and H₂, the most abundant molecules in K4-47 are CCH and CN, which have abundances of f ∼ 8 × 10^-7, relative to molecular hydrogen. Surprisingly, the next most abundant molecule is CH₃CCH, which has f ∼ 6 × 10^-7, followed by HCN with an abundance of ∼5 × 10^-7. The results suggest that K4-47 is the most chemically complex planetary nebula currently known. The molecular content of K4-47 closely resembles that of the C-star IRC+10216, but with lower abundances, except for HCO⁺, H₂CO, and CH₃CCH. The PN also chemically and morphologically resembles the bipolar protoplanetary nebula CRL 618, with similar enrichments of ¹³C, ¹⁵N, and ¹⁷O, suggestive of an explosive process at the end of the asymptotic giant branch.