Ubiquitous Aromatic Carbon Chemistry at the Earliest Stages of Star
  Formation
  Benzonitrile ($c$-C$_6$H$_5$CN), a polar proxy for benzene ($c$-C$_6$H$_6$}),
has the potential to serve as a highly convenient radio probe for aromatic
chemistry, provided this ring can be found in other astronomical sources beyond
the molecule-rich prestellar cloud TMC-1 where it was first reported by McGuire
et al. in 2018. Here we present radio astronomical evidence of benzonitrile in
four additional pre-stellar, and possibly protostellar, sources: Serpens 1A,
Serpens 1B, Serpens 2, and MC27/L1521F. These detections establish benzonitrile
is not unique to TMC-1; rather aromatic chemistry appears to be widespread
throughout the earliest stages of star formation, likely persisting at least to
the initial formation of a protostar. The abundance of benzonitrile far exceeds
predictions from models which well reproduce the abundances of carbon chains,
such as HC$_7$N, a cyanpolyyne with the same heavy atoms, indicating the
chemistry responsible for planar carbon structures (as opposed to linear ones)
in primordial sources is favorable but not well understood. The abundance of
benzonitrile relative to carbon-chain molecules displays sizable variations
between sources within the Taurus and Serpens clouds, implying the importance
of physical conditions and initial elemental reservoirs of the clouds
themselves.
