Discovery of Interstellar 2-Cyanoindene (2-C9H7CN) in GOTHAM Observations of TMC-1

• Published in: Astrophysical journal. Letters Vol. 938; no. 2; p. L12
• Main Authors: Sita, Madelyn L., Changala, P. Bryan, Xue, Ci, Burkhardt, Andrew M., Shingledecker, Christopher N., Kelvin Lee, Kin Long, Loomis, Ryan A., Momjian, Emmanuel, Siebert, Mark A., Gupta, Divita, Herbst, Eric, Remijan, Anthony J., McCarthy, Michael C., Cooke, Ilsa R., McGuire, Brett A.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-10-2022; IOP Publishing
• Subjects: Aromatic hydrocarbons; Astrochemistry; Chemical abundances; Fourier transforms; Hydrocarbons; Indene; Isomers; Markov chains; Microwave spectroscopy; Molecular clouds; Polycyclic aromatic hydrocarbons; Reaction rates; Rotational spectra; Rotational spectroscopy; Spectral line lists; Spectroscopy; Substitutes; Wavelengths
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ac92f4

We present laboratory rotational spectroscopy of five isomers of cyanoindene (2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier transform microwave spectrometer operating between 6 and 40 GHz. Based on these measurements, we report the detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2-
C
9
H
7
CN
) in GOTHAM line survey observations of the dark molecular cloud TMC-1 using the Green Bank Telescope at centimeter wavelengths. Using a combination of Markov Chain Monte Carlo, spectral stacking, and matched filtering techniques, we find evidence for the presence of this molecule at the 6.3
σ
level. This provides the first direct observation of the ratio of a cyano-substituted polycyclic aromatic hydrocarbon to its pure hydrocarbon counterpart, in this case indene, in the same source. We discuss the possible formation chemistry of this species, including why we have only detected one of the isomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratio across this and other simpler species, as well as compare to those ratios predicted by astrochemical models. We conclude that while astrochemical models are not yet sufficiently accurate to reproduce absolute abundances of these species, they do a good job at predicting the ratios of hydrocarbon:CN-substituted species, further solidifying -CN tagged species as excellent proxies for their fully symmetric counterparts.