On the origin and evolution of isotopes of carbon, nitrogen, and oxygen

Calculations of CNO processing in stellar envelopes, based on theoretical nucleosynthesis rather than empirical abundances in evolving stars, are presented and used in two models for the chemical evolution of the solar neighborhood. Seven stable isotopes are considered: C-12, C-13, N-14, N-15, O-16, O-17, and O-18. The two models ('infall' and 'initial-burst') represent extremes of types consistent with general constraints and include theoretical estimates of other nucleosynthesis sites and yields for CNO isotopes. The results obtained are found to predict that all CNO isotopes are produced mainly by stars with lifetimes much less than the age of the Galaxy (even at the present time when low-mass stars have the greatest death rate), so that isotopic ratios evolve very slowly after the first few billion years. Consequences of these slow changes are that the isotopic ratios cannot be employed to test between alternative hypotheses and that galactic evolution does not seem to be able to account for the apparent difference between the C-13/C-12 ratio in the solar system and in molecular clouds. The predicted envelope processing is shown to lead to approximately the solar-system values for the C-13/C-12 and O-17/O-16 abundance ratios but to a N-14/C-12 ratio that is too small by at least a factor of 2.