Lithium abundance in atmospheres of F- and G-type supergiants and bright giants

Abstract

We present a non-local thermodynamic equilibrium abundance analysis of the resonance doublet Li I at 6707.8 A for 55 Galactic F and G supergiants and bright giants. The derived lithium abundances log ε(Li) may be considered in three groups, namely: (i) 10 Li-rich giants with log ε(Li) = 2.0-3.2 (all 10 are F-type or A9 stars); (ii) 13 G- to K0-type stars with Li abundances in the narrow range log ε(Li) = 1.1-1.8; (iii) all other stars provide just upper limits to the Li abundance. The derived Li abundances are compared with theoretical predictions of 2-15 Msun stars. Our results are generally in good agreement with theory. In particular, the absence of detectable lithium for the majority of programme stars is explainable. The comparison suggests that the stars may be separated by mass M into two groups, namely M < 6 Msun and M > 6 Msun. All Li-rich giants and supergiants with log ε(Li) > 2.0 have masses M < 6 Msun; 11 of 13 stars with log ε(Li) = 1.1-1.8, specifically the stars with M < 6 Msun, show good agreement with the post-first dredge-up surface abundance log ε(Li) = 1.4 predicted for the non-rotating 2-6 Msun stellar models. An absence of Li-rich stars in the range M > 6 Msun agrees with the theoretical prediction that F and G supergiants and giants with M > 6 Msun cannot show detectable lithium. We note that present theory appears unable to account for the derived Li abundances for some stars, namely for (i) a few relatively low-mass Li-rich giants (M < 6 Msun), whose high Li abundances accompanied by rather high rotational velocities or substantial nitrogen excess contradict theoretical predictions; (ii) the relatively high-mass supergiants HR 461 and HR 8313 (M > 6 Msun) with the detected abundances log ε= 1.3-1.5. It is possible that the lithium in such stars was synthesized recently.