Discovery of Interstellar 2-Cyanoindene (2-C$_9$H$_7$CN) in GOTHAM Observations of TMC-1

TL;DR

This study reports the first detection of interstellar 2-cyanoindene in TMC-1, combining laboratory spectroscopy and advanced data analysis to explore the chemistry of cyano-substituted polycyclic aromatic hydrocarbons in space.

Contribution

It provides the first direct observation of a cyano-substituted PAH in space and compares its abundance ratio to theoretical predictions, advancing astrochemical understanding.

Findings

Detection of 2-cyanoindene at 6.3σ significance in TMC-1.

First measurement of the ratio of cyano-PAH to hydrocarbon PAH in space.

Astrochemical models predict ratios well but not absolute abundances.

Abstract

We present laboratory rotational spectroscopy of five isomers of cyanoindene (2-, 4-, 5-, 6-, and 7-cyanoindene) using a cavity Fourier-transform microwave spectrometer operating between 6-40 GHz. Based on these measurements, we report the detection of 2-cyanoindene (1H-indene-2-carbonitrile; 2-C$_9$H$_7$CN) in GOTHAM line survey observations of the dark molecular cloud TMC-1 using the Green Bank Telescope at centimeter wavelengths. Using a combination of Markov Chain Monte Carlo (MCMC), spectral stacking, and matched filtering techniques, we find evidence for the presence of this molecule at the 6.3$\sigma$ level. This provides the first direct observation of the ratio of a cyano-substituted polycyclic aromatic hydrocarbon (PAH) to its pure hydrocarbon counterpart, in this case indene, in the same source. We discuss the possible formation chemistry of this species, including why we have only detected one of the isomers in TMC-1. We then examine the overall hydrocarbon:CN-substituted ratio across this and other simpler species, as well as compare to those ratios predicted by astrochemical models. We conclude that while astrochemical models are not yet sufficiently accurate to reproduce absolute abundances of these species, they do a good job at predicting the ratios of hydrocarbon:CN-substituted species, further solidifying -CN tagged species as excellent proxies for their fully-symmetric counterparts.