Carbon isotope ratios along the giant branch of M67

The carbon isotope ratio in the giant branch stars of the open cluster M67 was studied in order to determine at what stage in the evolutionary sequence a star departs from standard evolution and undergoes excess surface mixing that has been observed in low-mass stars. The results show that the subgiants seem to have undergone little or no mixing. The lower giant branch stars exhibit normal 'fist dredge-up' mixing ratios. The 'tip' giants and the 'clump' giants exhibit ratios considerably lower than predicted by standard mixing theories. Mixing on the giant branch caused by meridional circulation currents due to stellar rotation could have caused additional CN-processed elements to be mixed to the surface. This mixing would have occurred after the standard mixing phase and before the He-core flash. 29 refs.