Since 13C is believed to be produced by non-equilibrium CNO processing in stellar evolution (Truran 1977), measurements of the carbon ratio Rc ≡ [12C] / [13C] in the interstellar medium may provide important information on nucleo-synthesis. Commonly, the ratio (N13/N18)LTE ≡ [13CO/C18O]LTE is measured and from this RLTE ≡ [12CO/13CO]LTE is deduced and these values are often identified with Rc. However, this line of reasoning can be misleading for two reasons (Dickman et al. 1979):
(1) The difficulty of determining accurate column densities, [13C16O] and [12C18O], because of the complexity of the radiative transfer problem;
(2) The possible role of fractionation, whereby RCO ≡ [12CO] / [13CO] does not necessarily reflect the initial atomic abundance ratio RC (Watson et al. 1976, Langer 1977, Liszt 1978).