Isotopic Abundances of Carbon and Oxygen in Oxygen-Rich Giant Stars

Abstract

16O/17O and 12C/13C ratios in 23 M giants are determined from high resolution IR spectra observed with the KPNO FTS. The masses of our sample are estimated with the use of the evolutionary tracks by Claret (2004), which could account for only about half of our sample. The resulting rather large variation of 16O/17O in low mass stars is well consistent with the prediction of the evolutionary models, but quite low 16O/17O ratios observed in some higher mass stars cannot be explained with the model prediction. The observed 12C/13C ratios are mostly around 10, in contradiction with the predict 12C/13C ratios of about 20. Thus we confirm a long-standing 12C/13C puzzle, and it appears that this dilemma may not be resolved yet even with extra mixing such as "cool bottom processing" expected only in low mass stars.