EVOLUTION OF OH AND CO-DARK MOLECULAR GAS FRACTION ACROSS A MOLECULAR CLOUD BOUNDARY IN TAURUS

We present observations of (CO)-C-12 J = 1-0, (CO)-C-13 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]/[H-2] decreases from 8 x 10(-7) to 1 x 10(-7) as A, increases from 0.4 to 2.7 mag following an empirical law: [OH]/[H-2] = 1.5 x 10(-7) + 9.0 x 10(-7) x 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 x 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 H-2 and drops at high visual extinction due to CO formation. The DGF peaks in the extinction range where H-2 has already formed and achieved self-shielding but (CO)-C-12 has not. Two narrow velocity components with a peak-to-peak spacing of similar to 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.