The /C-13/O/C-18/O ratio in interstellar dark clouds - Evidence for isotopic fractionation

Astronomy and Astrophysics – Astrophysics

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Abundance, Carbon Isotopes, Carbon Monoxide, Interstellar Chemistry, Interstellar Matter, Isotope Separation, Chemical Fractionation, Visual Observation

Scientific paper

Measurements of the double ratio 13C16O/12C18O have been made in L1524, L183, L204, and the region around IC 5146. A comparison of the ratio RLTE ≡ {[12CO]/[13CO]}LTE (inferred using a terrestrial value for [16]/[18O]) with visual extinction, Aυ, shows that values of RLTE are generally lower than terrestrial and more scattered for Aυ ≤ 3.5 mag. Above this value the ratios are more uniform and consistent with the terrestrial value 89. These results are most plausibly attributed to the effects of isotope fractionation. CO observations in the vicinity of IC 5146, where a copious UV flux is present, suggest that fractionation is effective, as might be expected. However, this interpretation is difficult to substantiate unequivocally.
We conclude that RCO = [12CO]/[13CO] is statistically consistent with a terrestrial value (assuming the oxygen ratio to be terrestrial) in the clouds studied, at positions where the visual extinction Aυ ≥ 3.5 mag. At positions where the extinction is low, fractionation is effective in lowering RCO. These results imply a terrestrial value for the carbon-isotope ratio: RC ≡ {12C}/{13C}.

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