The Lithium Isotope Ratio in F and G Field Stars

Abstract

Theoretical profiles of the resonance line of Li I have been computed using an absorption coefficient which is the sum of that for each of the four components and a model stellar atmosphere. These profiles have been used to verify the lithium abundances derived by previous investigators with various approximations. A study of the feasibility of measuring the lithium isotope ratio with high dispersion photographic spectra was made, with negative results. We obtain profiles of λ6708 A of Li I, λ6717 A Ca I, and sometimes λ6710.3 A of Fe I, and the D lines of Na I for a selected group of F, G, and early K field stars. These profiles were observed with the Fabry-Perot interferometer at the coude focus of the 100 telescope with a resolution of 0.06-0.09 A. Extensive checks of the accuracy of the wavelength scale, the instrumental profile, and the parasitic light level are made, including observations of the lunar spectrum. The D line profiles were used to test the accuracy of the computed theoretical profiles. The observed stellar line profiles are fit by calculated ones to determine the lithium abundance, calcium abundance, rotational velocity, radial velocity, microturbulent velocity, and lithium isotope ratio for the program stars. The rotational velocities are all greater than 2 km/sec, and range up to 10 km/sec. The microturbulent velocities in the program stars must be greater than zero. The isotope ratio Li⁶/Li⁷, determined by two independent methods, is zero in all the stars, with the possible exception of χ' Ori, for which we obtain Li⁶/Li⁷ = 0.1 (±0.1).