Unveiling the chemistry of interstellar CH: Spectroscopy of the 2 THz
  $N=2\leftarrow 1$ ground state line
  The methylidyne radical CH is commonly used as a proxy for H$_2$ in the cold,
neutral phase of the interstellar medium. The optical spectroscopy of CH is
limited by interstellar extinction, whereas far-infrared observations provide
an integral view through the Galaxy. While the HF ground state absorption,
another H$_2$ proxy in diffuse gas, frequently suffers from saturation, CH
remains transparent both in spiral-arm crossings and high-mass star forming
regions, turning this light hydride into a universal surrogate for H$_2$.
However, in slow shocks and in regions dissipating turbulence its abundance is
expected to be enhanced by an endothermic production path, and the idea of a
"canonical" CH abundance needs to be addressed. The $N=2\leftarrow 1$ ground
state transition of CH at $\lambda 149\,\mu$m has become accessible to
high-resolution spectroscopy thanks to GREAT aboard SOFIA. Its unsaturated
absorption and the absence of emission makes it an ideal candidate for the
determination of column densities with a minimum of assumptions. Here we
present an analysis of four sightlines towards distant, far-infrared bright
Galactic star forming regions. If combined with the sub-millimeter line of CH
at $\lambda 560\,\mu$m, environments forming massive stars can be analyzed. For
this we present a case study on the "proto-Trapezium" cluster W3 IRS5, and
demonstrate that the sub-millimeter/far-infrared lines of CH reliably trace not
only diffuse but also dense, molecular gas. While we confirm the global
correlation between the column densities of HF and those of CH, clear signposts
of an over-abundance of CH are observed towards lower densities. A quiescent
ion-neutral chemistry alone cannot account for this over-abundance. Vortices
forming in turbulent, diffuse gas may be the setting for an enhanced production
path.
