Evolution of OH and CO-dark Molecular Gas Fraction Across a Molecular
  Cloud Boundary In Taurus
  We present observations of 12CO J=1-0, 13CO J=1-0, HI, and all four
ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary
region of the Taurus molecular cloud. Based on a PDR model that reproduces CO
and [CI] emission from the same region, we modeled the three OH transitions,
1612, 1665, 1667 MHz successfully through escape probability non-LTE radiative
transfer model calculations. We could not reproduce the 1720 MHz observations,
due to un-modeled pumping mechanisms, of which the most likely candidate is a
C-shock. The abundance of OH and CO-dark molecular gas (DMG) are well
constrained. The OH abundance [OH]/[H2] decreases from 8*10-7 to 1*10-7 as Av
increases from 0.4 to 2.7 mag, following an empirical law [OH]/[H2]= 1.5 *
10^{-7} + 9.0 * 10^{-7} * exp(-Av/0.81), which is higher than PDR model
predictions for low extinction regions by a factor of 80. The overabundance of
OH at extinctions at or below 1 mag is likely the result of a C-shock. The dark
gas fraction (DGF, defined as fraction of molecular gas without detectable CO
emission) decreases from 80% to 20%, following a gaussian profile DGF= 0.90 *
exp(-( Av -0.79 )/0.71)^2) This trend of the DGF is consistent with our
understanding that the DGF drops at low visual extinction due to
photodissociation of H2 and drops at high visual extinction due to CO
formation. The DGF peaks in the extinction range where H2 has already formed
and achieved self-shielding but 12CO has not. Two narrow velocity components
with a peak-to-peak spacing of ~ 1 km s-1 were clearly identified. Their
relative intensity and variation in space and frequency suggest colliding
streams or gas flows at the boundary region.
