Detection of Interstellar HC${}_4$NC and an Investigation of Isocyanopolyyne Chemistry under TMC-1 Conditions

Abstract

We report an astronomical detection of HC${}_4$NC for the first time in the interstellar medium with the Green Bank Telescope toward the TMC-1 molecular cloud with a minimum significance of $10.5\sigma$. The total column density and excitation temperature of HC${}_4$NC are determined to be $3.29_{-1.20}^{+8.60}\times 10^{11}$ cm${}^{-2}$ and $6.7_{-0.3}^{+0.3}$ K, respectively, using the MCMC analysis. In addition to HC${}_4$NC, HCCNC is distinctly detected whereas no clear detection of HC${}_6$NC is made. We propose that the dissociative recombination of the protonated cyanopolyyne, HC${}_5$NH${}^{+}$, and the protonated isocyanopolyyne, HC${}_4$NCH${}^{+}$, are the main formation mechanisms for HC${}_4$NC while its destruction is dominated by reactions with simple ions and atomic carbon. With the proposed chemical networks, the observed abundances of HC${}_4$NC and HCCNC are reproduced satisfactorily.

Accepted in the Astrophysical Journal Letters

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• Discovery of the Pure Polycyclic Aromatic Hydrocarbon Indene ($c$-C${}_9$H${}_8$) with GOTHAM Observations of TMC-1
• Discovery of Interstellar 2-Cyanoindene (2-C${}_9$H${}_7$CN) in GOTHAM Observations of TMC-1
• Early Science from GOTHAM: Project Overview, Methods, and the Detection of Interstellar Propargyl Cyanide (HCCCH${}_2$CN) in TMC-1
• Discovery of HC4NC in TMC-1: A study of the isomers of HC3N, HC5N, and HC7N
• Discovery of the acetyl cation, CH3CO+, in space and in the laboratory
• Astronomical Detection of the Interstellar Anion C10H- towards TMC-1 from the GOTHAM Large Program on the GBT
• Machine Learning of Interstellar Chemical Inventories
• Detection of Interstellar $E$-1-cyano-1,3-butadiene in GOTHAM Observations of TMC-1
• Discovery of Interstellar trans-cyanovinylacetylene (HCCCH=CHCN) and vinylcyanoacetylene (H${}_2$C=CHC${}_3$N) in GOTHAM Observations of TMC-1
• Chemical variations across the TMC-1 boundary: molecular tracers from translucent phase to dense phase