CH as a Molecular Gas Tracer and C-Shock Tracer Across a Molecular Cloud
  Boundary in Taurus
  We present new observations of all three ground-state transitions of the
methylidyne (CH) radical and all four ground-state transitions of the hydroxyl
(OH) radical toward a sharp boundary region of the Taurus molecular cloud.
These data were analyzed in conjunction with existing CO and dust images. The
derived CH abundance is consistent with previous observations of translucent
clouds ($0.8\le A_{v} \le 2.1$ mag). The $X({\rm CH})$-factor is nearly a
constant at $(1.0\pm0.06)\times 10^{22}$ $\rm {cm^{-2}~K^{-1}~km^{-1}~s}$ in
this extinction range, with less dispersion than that of the more widely used
molecular tracers CO and OH. CH turns out be a better tracer of total column
density in such an intermediate extinction range than CO or OH. Compared with
previous observations, CH is overabundant below 1 mag extinction. Such an
overabundance of CH is consistent with the presence of a C-shock. CH has two
kinematic components, one of which shifts from 5.3 to 6 km s$^{-1}$, while the
other stays at 6.8 km s$^{-1}$ when moving from outside toward inside of the
cloud. These velocity behaviors exactly match with previous OH observation. The
shifting of the two kinematic components indicates colliding streams or gas
flow at the boundary region, which could be the cause of the C-shock.
