Discovery of Interstellar trans-cyanovinylacetylene (HCCCH=CHCN) and vinylcyanoacetylene (H$_2$C=CHC$_3$N) in GOTHAM Observations of TMC-1

TL;DR

This paper reports the discovery of two unsaturated organic molecules in the interstellar medium of TMC-1 using advanced radio telescope observations and analysis techniques, providing insights into their formation and abundance.

Contribution

The study presents the first detection of trans-(E)-cyanovinylacetylene and vinylcyanoacetylene in TMC-1, with detailed analysis of their abundances and formation mechanisms, expanding knowledge of interstellar organic chemistry.

Findings

Detected two new interstellar organic molecules.

Derived their column densities and compared with thermochemical models.

Provided upper limits for related species, refining chemical formation pathways.

Abstract

We report the discovery of two unsaturated organic species, trans-(E)-cyanovinylacetylene and vinylcyanoacetylene, using the second data release of the GOTHAM deep survey towards TMC-1 with the 100 m Green Bank Telescope. For both detections, we performed velocity stacking and matched filter analyses using Markov chain Monte Carlo simulations, and for trans-(E)-cyanovinylacetylene, three rotational lines were observed at low signal-to-noise (${\sim}$3$\sigma$). From this analysis, we derive column densities of $2\times10^{11}$ and $3\times10^{11}$ cm$^{-2}$ for vinylcyanoacetylene and trans-(E)-cyanovinylacetylene, respectively, and an upper limit of $<2\times10^{11}$ cm$^{-2}$ for trans-(Z)-cyanovinylacetylene. Comparisons with G3//B3LYP semi-empirical thermochemical calculations indicate abundances of the [H$_3$C$_5$N}] isomers are not consistent with their thermodynamic stability, and instead their abundances are mainly driven by dynamics. We provide discussion into how these species may be formed in TMC-1, with reference to related species like vinyl cyanide (CH$_2$=CHCN). As part of this discussion, we performed the same analysis for ethyl cyanide (CH$_3$CH$_2$CN), the hydrogenation product of CH$_2$=CHCN. This analysis provides evidence -- at 4.17$\sigma$ significance -- an upper limit to the column density of $<4\times10^{11}$ cm$^{-2}$; an order of magnitude lower than previous upper limits towards this source.