CARBON ISOTOPE RATIOS AND LITHIUM ABUNDANCES IN OLD DISK GIANTS

New very-high-resolution, low-noise CCD spectra at 8000 and 6700 angst rom have been obtained for stars of a previously studied sample of old disk giants. Carbon isotope ratios have been derived from CN red syst em features appearing on the 8000 angstrom spectra, and lithium abunda nces have been derived from the Li I resonance doublet at 6708 angstro m. The lithium abundances have been used to assist in the discriminati on of true old disk giants from those that may belong to the young dis k population. The carbon isotope ratios of the old disk stars exhibit a pronounced dependence on stellar luminosity, from C-12/C-13 greater- than-or-equal-to 40 in stars with M(V) greater-than-or-equal-to + 1.8, to C-12/C-13 congruent-to 13 in stars with M(V) < + 1.8. Comparisons with cluster color-magnitude diagrams suggest that most of the sample stars are on their first ascent of the giant branch. The lower-luminos ity giants with high carbon isotope ratios have not experienced signif icant convective envelope mixing of CN-cycle fusion products to their surfaces; this agrees qualitatively and quantitatively with standard s tellar-evolution theory. The large change in C-12/C-13 ratios that set s in abruptly at M(V) is similar to + 1.8 appears to be due mainly to the first dredge-up. The observed absolute magnitude of this event is in reasonable accord with standard theory, but the amount of decrease in the isotope ratio is far more than predicted by first dredge-up com putations.