The cool giant HD 77361—a super Li-rich star

Super Li-rich stars form a very small and enigmatic group whose existence cannot be explained in terms of the standard stellar evolution theory. The goal of our study is to check the reality of this group of cool giants based on an independent technique. We have carried out such a check using the K giant HD 77361 (HR 3597), which has previously been assigned to this rare type, as an example. We have redetermined the effective temperature T eff and surface gravity log g for this star. We have applied two different methods, photometric and spectroscopic, to estimate T eff (the accuracy of the Li-abundance determination depends significantly on this parameter). The value of log g has been found from the highly accurate parallax of this nearby star. To apply the photometric method of determining T eff , we have performed U BV observations of the star, which yielded V = 6.18 ± 0.03, B − V = 1.13 ± 0.01, and U − B = 1.18±0.05. The following parameters of the star have been found: effective temperature T eff = 4370± 100 K, surface gravity log g = 2.30 ± 0.10, iron abundance log ϵ (Fe) = 7.49 ± 0.14, microturbulence V t = 1.1 ± 0.2 km s −1 , rotational velocity V sin i = 4.5 km s −1 , and mass M = 1.3 ± 0.2 M ⊙ The lithium abundance has been determined from a non-LTE analysis of three Li I lines: the resonance line at 6707.8 Å and the subordinate lines at 6103.6 and 8126.4 Å (the latter in a blend with a CN molecular line). We have found a high lithium abundance, log ϵ (Li) = 3.75 ± 0.11, which exceeds considerably the initial abundance log ϵ (Li) = 3.2 ± 0.1 for young stars in the solar neighborhood. Thus, we have confirmed that the K giant HD 77361 actually belongs to the type of super Li-rich stars. It is noted that a high lithium abundance in such cool giants is inconsistent with predictions of the standard stellar evolution theory and may suggest a recent synthesis of lithium in these stars.