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, H i, and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. Based on a photodissociation region (PDR) model that reproduces CO and [C i] emission from the same region, we modeled the three OH transitions, 1612, 1665, and 1667 MHz successfully through escape probability non-local thermal equilibrium radiative transfer model calculations. We could not reproduce the 1720 MHz observations, due to unmodeled pumping mechanisms, of which the most likely candidate is a C-shock. The abundance of OH and CO-dark molecular gas is well-constrained. The OH abundance [OH]/[H2] decreases from 8 × 10 − 7 ?> to 1 × 10 − 7 ?> as A v increases from 0.4 to 2.7 mag following an empirical law: [ OH ] / [ H 2 ] = 1.5 × 10 − 7 + 9.0 × 10 − 7 × exp ( − A v / 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 the fraction of molecular gas without detectable CO emission) decreases from 80% to 20% following a Gaussian profile: DGF = 0.90 × exp − A v − 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.