Interstellar Detection of 2-cyanocyclopentadiene, C<sub>5</sub>H<sub>5</sub>CN, a Second Five-membered Ring toward TMC-1
Kin Long Kelvin Lee, P. Bryan Changala, Ryan A. Loomis, Andrew M. Burkhardt, Ci Xue, Martin Cordiner, Steven B. Charnley, Michael McCarthy, Brett A. McGuire
Abstract
Abstract Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov Chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, a low-lying isomer of 1-cyanocyclopentadiene, which was recently discovered there by the same methods. The new observational data also yield a considerably improved detection significance for the more stable isomer and evidence for several individual transitions between 23–32 GHz. Through our MCMC analysis, we derive cospatial, total column densities of 8.3 × 10 11 and 1.9 × 10 11 cm −2 for 1- and 2-cyanocyclopentadiene, respectively, corresponding to a ratio of ∼4.4 favoring the former. The derived abundance ratios point toward a common formation pathway—most likely being cyanation of cyclopentadiene by analogy to benzonitrile.