Discovery of the Pure Polycyclic Aromatic Hydrocarbon Indene ($c$-C$_9$H$_8$) with GOTHAM Observations of TMC-1

TL;DR

This paper reports the first interstellar detection of the pure hydrocarbon PAH indene in TMC-1, revealing its abundance exceeds model predictions and opening new avenues for understanding PAH formation in space.

Contribution

It provides the first detection of a pure hydrocarbon PAH in space and highlights discrepancies between observed and modeled abundances, suggesting incomplete formation pathways.

Findings

Indene is the most abundant organic ring detected in TMC-1.

Observed indene abundance exceeds model predictions by several orders of magnitude.

Detection suggests related aromatic species may also be detectable in dark clouds.

Abstract

Polycyclic Aromatic Hydrocarbons (PAHs) have long been invoked in the study of interstellar and protostellar sources, but the unambiguous identification of any individual PAH has proven elusive until very recently. As a result, the formation mechanisms for this important class of molecules remain poorly constrained. Here we report the first interstellar detection of a pure hydrocarbon PAH, indene (C$_9$H$_8$), as part of the GBT Observations of TMC-1: Hunting for Aromatic Molecules (GOTHAM) survey. This detection provides a new avenue for chemical inquiry, complementing the existing detections of CN-functionalized aromatic molecules. From fitting the GOTHAM observations, indene is found to be the most abundant organic ring detected in TMC-1 to date. And from astrochemical modeling with NAUTILUS, the observed abundance is greater than the model's prediction by several orders of magnitude suggesting that current formation pathways in astrochemical models are incomplete. The detection of indene in relatively high abundance implies related species such as cyanoindene, cyclopentadiene, toluene, and styrene may be detectable in dark clouds.