Carbon isotope ratios and lithium abundances in open cluster giants

Abstract

New high-resolution, high S/N spectra of CN lines at 8000 A and Li lines at 6707 A have been obtained for giants in about 20 Galactic open clusters and C-12/C-13 ratios and Li abundances determined for these stars. The ages of the clusters vary from about 50 million years to 5 billion years, and their turn-off masses vary from 1 solar mass to about 6 solar masses. The ages and turn-off masses were determined by fitting theoretical isochrones to the cluster color-magnitude diagrams. Correlation of the isotope ratio and the Li abundances with the cluster turn-off masses indicates the following: (1) the C-12/C-13 ratio increases steeply with the turn-off mass until a mass of approximately 2.2 solar masses when the ratio levels off abruptly to a value near 26; (2) older clusters with turn-off masses lower than about 2.2 solar masses in general exhibit C-12/C-13 ratios that are considerably lower than the theoretically predicted values while those with larger turn-off masses show ratios close to standard predictions; and (3) no strong correlation exists between the Li abundances and the cluster turn-off mass, and the Li abundances in giants are, in general, lower than theoretically predicted values. Various theories to explain the observed abundance trends are discussed.