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 (${{{\rm{H}}}_{2}}^{13}$CO, c-13CCCH2, c-CC13CH2, ${{\mathrm{CH}}_{3}}^{13}$CN, 13CH3CN, ${{\mathrm{CH}}_{3}}^{13}$CCH, 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.