Lithium in F and G type stars - I. Isotope abundances.

The wavelength of the lithium 6708 A line has been determined from 6 spectra at A mm-1 for 12 lithium-rich F and G type stars. Li7 is always the predominant isotope. The results are consistent with an upper limit of about 0.5 for R', the Li6/Li7 ratio. There is some suggestion of two groups of stars one with R' 0 and another with R' I /3. Stars of particular interest, all of which have R' 0 are (I) the two subgiant components of the visual binary yCrA (2) the highly evolved G8 subgiant a Sgr (3) the slightly evolved, metal deficient, subdwarf y Pav. Dor (F8, slightly above the main sequence) appears to have a significant amount of Li6 (R' o ). It also has a very active chromosphere as shown by H and K emission and A5 876 (He I, D3) absorption. A review of the available data indicates that the few stars with high R' ( o ) are all subgiants. Radial velocity determinations for these and some other stars are given in the appendix.