On the observational characteristics of lithium-enhanced giant stars in comparison with normal red giants

Abstract

While lithium is generally deficient in the atmosphere of evolved giant stars because of the efficient mixing-induced dilution, a small fraction of red giants show unusually strong Li lines indicative of conspicuous abundance excess. With an aim to shed light on the origin of these peculiar stars, we carried out a spectroscopic study on the observational characteristics of selected 20 bright giants already known to be Li-rich from past studies, in comparison with the reference sample of a large number of normal late G -early K giants. Our special attention was paid to clarifying any difference between the two samples from a comprehensive point of view (i.e., with respect to stellar parameters, rotation, activity, kinematic properties, 6 Li/ 7 Li ratio, and the abundances of Li, Be, C, O, Na, S, and Zn). Our sample stars are roughly divided into "bump/clump group" and "luminous group" according to the positions on the HR diagram. Regarding the former group (1.5 < ∼ log(L/L ⊙ ) < ∼ 2 and M ∼1.5-3 M ⊙ ), Li-enriched giants and normal giants appear practically similar in almost all respect except for Li, suggesting that surface Li enhancement in this group may be a transient episode which normal giants undergo at certain evolutionary stages in their lifetime. Meanwhile, those Li-rich giants belonging to the latter group (log(L/L ⊙ ) ∼ 3 and M ∼3-5 M ⊙ ) appear more anomalous in the sense that they tend to show higher rotation as well as higher activity, and that their elemental abundances (especially those derived from high-excitation lines) are apt to show apparent overabundances, though this might be due to a spurious effect reflecting the difficulty of abundance derivation in stars of higher rotation and activity. Our analysis confirmed considerable Be deficiency as well as absence of 6 Li as the general characteristics of Li-rich giants under study, which implies that engulfment of planets is rather unlikely for the origin of Li-enrichment.

Concerning the Li-rich status of KIC 9821622: a Kepler field RGB star reported as a Li-rich giant

“…Our derived values are similar to the values derived by Jofré et al (2015) and infrared APOGEE spectra (Majewski et al 2017;García Pérez et al 2016 shown in Figure 1 suggest stellar mass is close to 1.1 M which is significantly less than the mass = 1.62 M derived from asteroseismology. However, higher T eff = 4890 K derived by Yong et al (2016); Takeda & Tajitsu (2017) yield mass which is close to the value derived from asteroseismology. In either case, star's position is below the luminosity bump (See Figure 1).…”

Abundance analyses of Li-enriched and normal giants in the GALAH survey

“…Surface Be abundance of a giant is diluted by the first dredge-up and replenished by ingestion of circumstellar material. The point that Be has not increased above first dredge-up values in Li enriched stars has been made previously (Melo et al 2005;Takeda & Tajitsu 2017). This is certainly a valid point against the external reservoir as a source of Li for Li-rich and especially for super Li-rich giants.…”

“…Although the literature on the compositions of Li enriched giants is now quite extensive, few studies have tackled a fair sample of the Li enriched giants and a comparable sample of normal giants in order to minimize systematic errors in the analyses. Among published studies, we note that by Takeda & Tajitsu (2017) whose sample contained 20 Li enriched giants including five super Li-rich and all but two giants with a Li abundance A(Li) ≥ 2.0. Takeda & Tajitsu (2017) obtained C, O. Na, S and Zn abundances by line selections and methods previously used for a large sample of normal giants.…”

Do Hertzsprung‐gap stars show any chemical anomaly?

“…Given that our task is to study the surface abundances of Li, C, N, O, and Na for 62 Hertzsprung‐gap stars, we invoke Li i 6708, C i 5380, O i 6156–8, N i 7468, and Na i 6161 lines as in Takeda & Tajitsu () (for Li, C, O, and Na) and Paper II (for C, N, and O). The determination procedures of abundances and related quantities (e.g., non‐local thermodynamic equilibrium [LTE] correction, uncertainties due to ambiguities of atmospheric parameters) are essentially the same as described in these papers, which consist of two consecutive steps.…”

“…We then analyzed such derived W values by using WIDTH9 to determine A N (NLTE [Non‐Local Thermodynamical Equilibrium] abundance) and A L (LTE abundance), from which the NLTE correction Δ(≡ A N − A L ) was further derived. We adopted Procyon as the standard star of abundance reference (except for Li, which should be compared with the solar system abundance of 3.31 as done in Takeda & Tajitsu ), because it is known to have practically the same abundance as the Sun. We thus define the relative abundance as [X/H] ≡ (star) – (Procyon) (X = C, N, O, and Na).…”