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arXiv · 2110.13152

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

Mark A. Siebert·Kin Long Kelvin Lee·Anthony J. Remijan·Andrew M. Burkhardt·Ryan A. Loomis·Michael C. McCarthy·Brett A. McGuire
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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.

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Mark A. Siebert, Kin Long Kelvin Lee, Anthony J. Remijan, Andrew M. Burkhardt, Ryan A. Loomis, Michael C. McCarthy, Brett A. McGuire. 2021-10-25. CH3-Terminated Carbon Chains in the GOTHAM Survey of TMC-1: Evidence of Interstellar CH3C7N. https://doi.org/10.3847/1538-4357/ac3238

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