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

Abstract

The hydrocarbons CCH and cC 3 H 2 have been detected at multiple positions distributed across the Helix, along with the CN, indicating that these molecules are present throughout this very old planetary nebula. The species were identified by observations of the N=1→0 transitions of CCH and CN and the J=2 1,2 →1 0,1 transition of cC 3 H 2 , conducted with the new Atacama Large Millimeter/submillimeter Array (ALMA) prototype 12 m antenna of the Arizona Radio Observatory at 3 mm in wavelength. Column densities of the three species were calculated to be N tot (CCH)∼(0.8-6.6)×10 12 cm −2 , N tot (c-C 3 H 2)∼(0.3-5.9)× 10 11 cm −2 , and N tot (CN)∼(0.7-7.4)×10 12 cm −2 , corresponding to fractional abundances, with respect to H 2 , of f(CCH)∼(1.1-8.8)×10 −7 , f (c-C 3 H 2)∼(0.4-7.9)×10 −8 , and f(CN)∼(0.9-9.9)×10 −7. Surprisingly, the abundances are comparable to those measured in younger planetary nebulae (PNe), indicating that predicted photodestruction processes for gas-phase molecules are inefficient in these objects, even over long timescales. The distributions of these two hydrocarbons in the Helix appear to be correlated, with a ratio of [CCH]/[c-C 3 H 2 ]∼5-80 and an average value of 29±6-remarkably similar to both the range ([CCH]/[c-C 3 H 2 ]∼11-53) and average value (28.2 ± 1.4) found in diffuse clouds. The abundances of CCH, cC 3 H 2 , and CN in the Helix are also factors of 10-100 greater than those measured in diffuse gas. These results suggest that molecular ejecta from PNe are a possible external source for polyatomic species found in diffuse clouds, including C 60 .

Formation of Interstellar C60 from Silicon Carbide Circumstellar Grains

“…C 60 is known to have an extreme longevity in high-energy photon environments (Cataldo et al 2009). Its high stability enables it to survive into the PN stage, and then be ejected, along with other molecular material, into the diffuse ISM, where it is also observed in ionized form (Berné et al 2017;Schmidt et al 2018). Our findings also suggest that other carbon nanostructures such as nanotubes and "buckybowls" may be created in significant quantities by SiC grain shock heating and are transported into the ISM.…”

A new radio molecular line survey of planetary nebulae

“…Due to the large angular diameter of NGC 7293, the IRAM telescope beam measured only a small portion of the extensive molecular region. As such, it is only possible to make a comparison with a nearby region (∆α = −435 , ∆γ = 75 ), as observed by , Schmidt & Ziurys (2017a), and Schmidt et al (2018). Their spectra revealed three CO velocity components at −11, −21, and −29 km s −1 and −13, −21, −29 km s −1 in HNC and HCN, respectively.…”

“…It is a relatively evolved object with a dynamical age of 12 000 yr (Schmidt & Ziurys 2017a), and contains a hard, point-like X-ray source (Guerrero et al 2001). Early radio molecular line observations were carried out by Huggins et al (1992) and Bachiller et al (1997), with many additional studies performed across the expansive molecular emission region of the PN (Young et al 1999;Zeigler et al 2013;Schmidt & Ziurys 2017a;Schmidt et al 2018).…”

“…produce a rich environment for molecular chemistry that has served as motivation for various radio molecular line surveys (e.g., Bachiller et al 1997;Zhang et al 2008;Schmidt et al 2018). Whereas the foregoing surveys were largely restricted to well-studied objects, the recent molecular line surveys of Schmidt & Ziurys (2016, 2017a extended the PN sample coverage to younger objects.…”

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

“…Recent millimeter observations of PNe, however, have shown the contrary, even into the late stages of nebular evolution. The Helix Nebula, one of the oldest known PNe (age ∼11,000 yr; Meaburn et al 2008), hosts a surprising variety of molecules with widespread distributions, including HCN, CN, HNC, HCO + , CCH, c-C 3 H 2 , and H 2 CO Schmidt & Ziurys 2017a;Schmidt et al 2018b). Toward the younger objects NGC 7027 and NGC 6537, species such as SO, H 2 CO, CS, N 2 H + , and even HC 3 N have been found (Zhang et al 2008;Edwards & Ziurys 2013).…”