1995A&A...302L..25M

The photon-dominated region (PDR) associated with the NGC 1977 molecular cloud has been observed at high resolution in the ¹³CO and C¹⁸O J=2-1 and the [CI] ³P₁-³P₀ lines. The N(¹³CO)/N(C¹⁸O) ratio has been plotted against visual extinction (A_ν) and fits a power law relation. The highest values, as expected, occur for observed positions with the lowest derived extinction, with N(¹³CO)/N(C¹⁸O) exceeding the terrestrial value (5.5) for A_ν≤60 magnitudes. In the outermost parts of the cloud (A_ν≤20 magnitudes) the N(¹³CO)/N(C¹⁸O) ratio is largest, up to 20. Comparison with similar observations of the S140 and Orion Bright Bar regions implies that higher incident UV field leads to increased fractionation effects. The N(CI)/N(CO) ratio has been plotted against visual extinction, as derived from N(C¹⁸O), for the range A_ν=20-100 magnitudes and also fits a power law relation. N(CI)/N(CO) increases with decreasing extinction from ∼0.03-0.04 for A_ν∼100 mags. to ≥0.1 for A_ν∼20 mags., corresponding to positions near the edge of the HII region/molecular cloud interface. Comparison with identical observations of the S140 and Orion Bright Bar regions implies that, unlike N(¹³CO)/N(C¹⁸O), the behaviour of N(CI)/N(CO) is insensitive to incident UV field over this range of extinction.