Carbon-to-nitrogen ratios along the evolutionary sequence of M67

A low-resolution spectroscopic study of the C and N abundances of stars in the old open cluster M67 was conducted in order to find the first dredge-up in the H-R diagram, when the surface C/N ratio is altered during evolution up the giant branch. The C/N ratio is found to change abruptly at the base of the giant branch. If probable systematic errors are allowed for, the C/N ratio does not change elsewhere on the cluster evolutionary sequence. The location in the H-R diagram of the onset of this change is in accordance with theoretical predictions of stellar evolution, but the composition change is complete at a substantially lower luminosity in the cluster than in standard models of red giant evolution, and the amount of the change in C/N is greater than that predicted by standard evolution calculations. 67 references.