CARBON ISOTOPE RATIOS ALONG THE GIANT BRANCH OF M67

We present a study of the carbon isotope ratio in the giant branch stars of the open cluster M67. The purpose of this project was 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 the past in low-mass stars. The sample stars in M67 range in evolutionary stages from subgiants to core He-burning or "clump" giants. We present here a high-resolution spectral analysis of these stars using spectrum synthesis of the CN red system near 8000 angstrom. The major result emerging from this study is that stars near the lower giant branch exhibit carbon isotope ratios that are compatible with standard "first dredge-up" mixing but stars at the tip of the giant branch and the clump giants exhibit anomalously low ratios. These low ratios were probably caused by excess mixing that occurred at some point between the first dredge-up and the He-core-flash stage. Mixing mechanisms that may have caused the low isotope ratios are discussed.