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  1. Abstract

    Recent detections of aromatic species in dark molecular clouds suggest that formation pathways may be efficient at very low temperatures and pressures, yet current astrochemical models are unable to account for their derived abundances, which can often deviate from model predictions by several orders of magnitude. The propargyl radical, a highly abundant species in the dark molecular cloud TMC-1, is an important aromatic precursor in combustion flames and possibly interstellar environments. We performed astrochemical modeling of TMC-1 using the three-phase gas-grain codeNAUTILUSand an updated chemical network, focused on refining the chemistry of the propargyl radical and related species. The abundance of the propargyl radical has been increased by half an order of magnitude compared to the previous GOTHAM network. This brings it closer in line with observations, but it remains underestimated by 2 orders of magnitude compared to its observed value. Predicted abundances for the chemically related C4H3N isomers within an order of magnitude of observed values corroborate the high efficiency of CN addition to closed-shell hydrocarbons under dark molecular cloud conditions. The results of our modeling provide insight into the chemical processes of the propargyl radical in dark molecular clouds and highlight the importance of resonance-stabilized radicals in polycyclic aromatic hydrocarbon formation.

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  2. Abstract We report the detection of the lowest-energy conformer of E -1-cyano-1,3-butadiene ( E -1- C 4 H 5 CN ), a linear isomer of pyridine, using the fourth data reduction of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) deep spectral survey toward TMC-1 with the 100 m Green Bank Telescope. We perform velocity stacking and matched-filter analyses using Markov chain Monte Carlo simulations and find evidence for the presence of this molecule at the 5.1 σ level. We derive a total column density of 3.8 − 0.9 + 1.0 × 10 10 cm −2 , which is predominantly found toward two of the four velocity components we observe toward TMC-1. We use this molecule as a proxy for constraining the gas-phase abundance of the apolar hydrocarbon 1,3-butadiene. Based on the three-phase astrochemical modeling code NAUTILUS and an expanded chemical network, our model underestimates the abundance of cyano-1,3-butadiene by a factor of 19, with a peak column density of 2.34 × 10 10 cm −2 for 1,3-butadiene. Compared to the modeling results obtained in previous GOTHAM analyses, the abundance of 1,3-butadiene is increased by about two orders of magnitude. Despite this increase, the modeled abundances of aromatic species do not appear to change and remain underestimated by one to four orders of magnitude. Meanwhile, the abundances of the five-membered ring molecules increase proportionally with 1,3-butadiene by two orders of magnitude. We discuss the implications for bottom-up formation routes to aromatic and polycyclic aromatic molecules. 
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  3. 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. 
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  4. Abstract 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. 
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  7. 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.

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