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Abstract Using data from the Green Bank Telescope (GBT) Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey, we report the first astronomical detection of the C 10 H − anion. The astronomical observations also provided the necessary data to refine the spectroscopic parameters of C 10 H − . From the velocity stacked data and the matched filter response, C 10 H − is detected at >9 σ confidence level at a column density of 4.04 − 2.23 + 10.67 × 10 11 cm −2 . A dedicated search for the C 10 H radical was also conducted toward TMC-1. In this case, the stacked molecular emission of C 10 H was detected at a ∼3.2 σ confidence interval at a column density of 2.02 − 0.82 + 2.68 × 10 11 cm −2 . However, as the determined confidence level is currently <5 σ , we consider the identification of C 10 H as tentative. The full GOTHAM data set was also used to better characterize the physical parameters including column density, excitation temperature, line width, and source size for the C 4 H, C 6 H, and C 8 H radicals and their respective anions, and the measured column densities were compared to the predictions from a gas/grain chemical formation model and from a machine learning analysis. Given the measured values, the C 10 H − /C 10 H column density ratio is ∼ 2.0 − 1.6 + 5.9 —the highest value measured between an anion and neutral species to date. Such a high ratio is at odds with current theories for interstellar anion chemistry. For the radical species, both models can reproduce the measured abundances found from the survey; however, the machine learning analysis matches the detected anion abundances much better than the gas/grain chemical model, suggesting that the current understanding of the formation chemistry of molecular anions is still highly uncertain. 

Abstract We report a systematic study of all known methyl carbon chains toward TMC-1 using the second data release of the GOTHAM survey, as well as a search for larger species. Using Markov Chain Monte Carlo simulations and spectral line stacking of over 30 rotational transitions, we report statistically significant emission from methylcyanotriacetylene (CH 3 C 7 N) at a confidence level of 4.6 σ , and use it to derive a column density of ∼10 11 cm −2 . We also searched for the related species, methyltetraacetylene (CH 3 C 8 H), and place upper limits on the column density of this molecule. By carrying out the above statistical analyses for all other previously detected methyl-terminated carbon chains that have emission lines in our survey, we assess the abundances, excitation conditions, and formation chemistry of methylpolyynes (CH 3 C 2 n H) and methylcyanopolyynes (CH 3 C 2 n -1 N) in TMC-1, and compare those with predictions from a chemical model. Based on our observed trends in column density and relative populations of the A and E nuclear spin isomers, we find that the methylpolyyne and methylcyanopolyyne families exhibit stark differences from one another, pointing to separate interstellar formation pathways, which is confirmed through gas–grain chemical modeling with nautilus . 

Unidentified infrared emission bands are ubiquitous in many astronomical sources. These bands are widely, if not unanimously, attributed to collective emissions from polycyclic aromatic hydrocarbon (PAH) molecules, yet no single species of this class has been identified in space. Using spectral matched filtering of radio data from the Green Bank Telescope, we detected two nitrile-group–functionalized PAHs, 1- and 2-cyanonaphthalene, in the interstellar medium. Both bicyclic ring molecules were observed in the TMC-1 molecular cloud. In this paper, we discuss potential in situ gas-phase PAH formation pathways from smaller organic precursor molecules.