Minchin, Nigel R. and White, Glenn J.
Molecular and atomic fractionation effects in the NGC 1977 molecular cloud.
Astronomy and Astrophysics, 302
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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 (Av) 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 Av≤60 magnitudes. In the outermost parts of the cloud (Av≤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 Av=20-100 magnitudes and also fits a power law relation. N(CI)/N(CO) increases with decreasing extinction from ~0.03-0.04 for Av ~100 mags. to ≥0.1 for Av ~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.
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