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

TL;DR

This study reports the first interstellar detection of HC$_4$NC in TMC-1, analyzes its abundance and formation mechanisms, and compares observed data with chemical network models to understand isocyanopolyyne chemistry.

Contribution

First detection of HC$_4$NC in space and investigation of its formation pathways under TMC-1 conditions.

Findings

HC$_4$NC detected with >10.5 sigma significance

HC$_4$NC and HCCNC abundances match chemical models

Proposed formation via dissociative recombination of protonated precursors

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 $3.29^{+8.60}_{-1.20}\times 10^{11}$ 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.