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

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 C 60 . 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 × 10 15 cm −2 , corresponding to fractional abundances with respect to H 2 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 C 60 . These results suggest that CCH may be a product of C 60 photodestruction, which is known to create C 2 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.

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

“…SiO is also a factor of 2 more abundant in M2-48 than K4-47, with f ∼2.9×10 −8 . K4-47 additionally has some of the highest abundances among PNe for HCN, CCH, HNC, and HCO + (Schmidt & Ziurys 2016, Schmidt & Ziurys 2017a, 2017b.…”

“…SiO and SO 2 have been detected in the oxygen-rich, middle-aged PN M2-48, as well as CN, HCN, HNC, CS, SO, HCO + , and N 2 H + . More recently, the presence of HCN, HCO + , HNC, and CCH has been established in 10 other PNe with varied morphologies and spanning a 10,000 yr age range (Schmidt & Ziurys 2016, 2017a, 2017b. In the odd 20 PNe studied at millimeter wavelengths, the most complex species detected thus far contain five atoms: HC 3 N and c-C 3 H 2 .…”

“…It has a hot central core traced by highly excited atomic lines such as [O III], with high-velocity lobes at the tips of the outflow, visible in [N II] and [S II] (e.g., Corradi et al 2000). For many years, the only molecular emission known in this object was from CO and H 2 (Lumsden et al 2001;Huggins et al 2005), but newer studies have revealed the presence of CS, HCO + , HCN, HNC, CCH, and even HC 3 N, suggesting a very carbon-rich object Schmidt & Ziurys 2016, 2017a, 2017bSchmidt et al 2018a). Perhaps more striking are the anomalous isotope ratios recently found in K4-47, with 12 C/ 13 C=2.2±0.8, 14 N/ 15 N=13.6±6.5, and 16 O/ 17 O= 21.4±10.3 (Schmidt et al 2018a).…”

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

“…There is a natural connection between these objects, as PNe supply most of the matter to the diffuse interstellar medium (ISM; Dorschner & Henning 1995). Furthermore, some PNe are known to contain C 60 (e.g., García-Hernández et al 2012), as well as a substantial number of other polyatomic molecules, including HCN, HNC, CCH, HCO + , H 2 CO, N 2 H + , and SO 2 (e.g., Zhang et al 2008;Schmidt & Ziurys 2016, 2017a, 2017b. The PN phase follows the asymptotic giant branch (AGB) and is the evolutionary path for most (lowand intermediate-mass) stars.…”

“…Several older nebulae contain polyatomic molecules with considerable abundances (e.g., Zeigler et al 2013;Schmidt & Ziurys 2017a). In fact, such abundances appear to remain fairly constant with the age of the nebulae across their life span of ∼10,000 years (Schmidt & Ziurys 2016, 2017a, 2017b. The molecular material could be "fossil" species of the former AGB shell and/or be created in the nebulae, possibly by gas-phase reactions or from fragmentation of larger molecules such as C 60 or even hydrogenated amorphous carbon (HACs; e.g., Duley et al 2015).…”