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