CH3-Terminated Carbon Chains in the GOTHAM Survey of TMC-1: Evidence of Interstellar CH3C7N

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\(\sigma\), and use it to derive a column density of \({\sim}\)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 (CH3C\(_{2n}\)H) and methylcyanopolyynes (CH3C\(_{2n-1}\)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 methylpolyynes and methylcyanopolyynes families exhibit stark differences from one another, pointing to separate interstellar formation pathways, which is confirmed through gas-grain chemical modeling with nautilus.