New Identifications of the CCH Radical in Planetary Nebulae: A Connection to C60?

Abstract

Abstract New detections of CCH have been made toward nine planetary nebulae (PNe), including K4–47, K3–58, K3–17, M3–28, and M4–14. Measurements of the N = 1 → 0 and N = 3 → 2 transitions of this radical near 87 and 262 GHz were carried out using the new 12 m and the Sub-Millimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). The presence of fine and/or hyperfine structure in the spectra aided in the identification. CCH was not observed in two PNe which are sources of C60. The planetary nebulae with positive detections represent a wide range of ages and morphologies, and all had previously been observed in HCN and HNC. Column densities for CCH in the PNe, determined from radiative transfer modeling, were N tot(CCH) ∼ 0.2–3.3 × 1015 cm−2, corresponding to fractional abundances with respect to H2 of f ∼ 0.2–47 × 10−7. The abundance of CCH was found to not vary significantly with kinematic age across a time span of ∼10,000 years, in contrast to predictions of chemical models. CCH appears to be a fairly common constituent of PNe that are carbon-rich, and its distribution may anti-correlate with that of C60. These results suggest that CCH may be a product of C60 photodestruction, which is known to create C2 units. The molecule may subsequently survive the PN stage and populate diffuse clouds. The distinct, double-horned line profiles for CCH observed in K3–45 and M3–28 indicate the possible presence of a bipolar flow oriented at least partially toward the line of sight.