A constraint on proposed explanations for low C-12/C-13 ratios in G-K giants

Some proposed explanations for the anomalously low C-12/C-13 ratios in G-K giants require initial main-sequence masses greater than some value M. The fraction of red giants which have initial masses greater than M is calculated theoretically as a function of M using self-consistent initial mass functions, birthrates, and stellar lifetimes. This fraction is compared with the actual fraction of G-K giants with anomalous C-12/C-13 ratios, derived from the observed distribution of C-12/C-13 ratios after corrections for selection effects. It is found that M must be less than around 1.5 solar masses in order to explain the observed number of anomalous stars. This constraint rules out meridional mixing or rapid mass loss on the main sequence as possible explanations for the low C-12/C-13 ratios. Shear instability in the core-envelope interface region and the He core flash appear to be the most promising alternatives.