Carbon and nitrogen abundances along the evolutionary sequence of M67

A spectroscopic study of stars in the old galactic star cluster M67 was made to find the first dredge-up, that stage of stellar evolution where the surface abundances of carbon and nitrogen are altered by convective mixing with material processed by the CN cycle in the stellar interior. A high-resolution study of three bright cluster stars was made to determine the oxygen abundance of the cluster. Techniques and linelists for large-scale spectrum synthesis of the CHA x X delta v = 0 and CN B x X delta v = 1 bands were developed and used to determine the C/N abundance ratio in giant and subgiant stars. Nineteen M67 stars were analyzed with this method. It was found that the C/N ratio changes abruptly at the elbow in the cluster Hertzsprung-Russell diagram; at (BxV)sub 0 = 0.94 the C/N ratio drops by about 0.6 dex between M_v approx. = + 3.5 and M_v approx. = + 2.8. If probable systematic errors are allowed for, the C/N ratio does not change elsewhere on the cluster evolutionary sequence. The location of this change in the Hertzsprung Russell diagram is in accordance with theoretical predictions of stellar evolution although the amount of change in C/N is greater than that predicted by standard evolution calculations.