Analysis of the Non-LTE Lithium Abundance for a Large Sample of F-, G-, and K-Giants and Supergiants

A five-dimensional interpolation method and corresponding computer program are developed for using published calculations to determine the non-LTE correction ∆_NLTE to the lithium abundance logε(Li) derived from the Li I 6707.8 Å line. The ∆_NLTE value is determined from the following five parameters: the effective temperature T_eff, the acceleration of gravity logg, the metallicity index [Fe/H], the microturbulent velocity V_t, and the LTE Li abundance logε(Li) . The program is used to calculate values of ∆_NLTE and the non-LTE Li abundance for 91 single bright giants from the list of Lebre, et al. By combining these results with data for 55 stars from the previous paper, we obtain the non-LTE values of logε(Li) for 146 FGK-giants and supergiants. We confirm that, because of the absence of the Li line in the spectra of most of these stars, it is only possible to estimate for them an upper bound for the Li abundance. A large spread is confirmed in logε(Li) for stars with masses M ≤ 6M _⦿ . A comparison of these results with model calculations of stars confirms the unique sensitivity of the lithium abundance to the initial rotation velocity V₀. We discuss the giants and supergiants with lithium abundances logε(Li) = 1.4 ± 0.3 , which could have a rotational velocity V₀=0 km/s and have already undergone deep convective mixing. Li-rich giants with lithium abundances logε(Li) ≥ 2 and nearly up to the initial value of logε(Li) = 3.2 ± 0.1 are examined. It is shown that the fraction of Li-rich giants with V₀ ≈ 0 – 50 km/s is consistent with current evolutionary models. The other stars of this type, as well as all of the “super Li-rich” giants, for which the standard theory is untenable, can be explained by invoking the hypothesis of recent lithium synthesis in the star or an alternative hypothesis according to which a giant planet is engulfed by the star.