LITHIUM IN THE UPPER CENTAURUS LUPUS AND LOWER CENTAURUS CRUX SUBGROUPS OF SCORPIUS-CENTAURUS

摘要

We utilize spectroscopically derived model atmosphere parameters and the Li i λ6104 subordinate line and the λ6708 doublet to derive lithium abundances for 12 members of the Upper Centaurus Lupus and Lower Centaurus Crux subgroups of the Scorpius-Centaurus OB Association. The results indicate any intrinsic Li scatter in our 0.9-1.4M⊙ stars is limited to ～0.15 dex, consistent with the lack of dispersion in ≥1.0M⊙ stars in the 100 Myr Pleiades and 30-50 Myr IC 2391 and 2602 clusters. Both ab initio uncertainty estimates and the derived abundances themselves indicate that the λ6104 line yields abundances with equivalent or less scatter than is found from the λ6708 doublet as a result of lower uncertainties for the subordinate feature, a result of low sensitivity to broadening in the subordinate feature. Because non-local thermodynamic equilibrium (NLTE) corrections are less susceptible to changes in surface gravity and/or metallicity for the 6104? line, the subordinate Li feature is preferred for deriving lithium abundances in young Li-rich stellar association stars with Teff ≥ 5200 K. At these temperatures, we find no difference between the Li abundances derived from the two Li i lines. For cooler stars, having temperatures at which main-sequence dwarfs show abundance patterns indicating overexcitation and overionization, the λ6104-based Li abundances are ～0.4 dex lower than those derived from the λ6708 doublet. The trends of the abundances from each feature with Teff suggest that this difference is due to (near)UV photoionization, which in NLTE preferentially ionizes Li atoms in the subordinate 2p state relative to the 2s resonance line state due to opacity effects. Consequently, this overionization of Li in the 2p state, apparently not adequately accounted for in NLTE corrections, weakens the λ6104 feature in cooler stars. Accordingly, the λ6708-based abundances may give more reliable estimates of the mean Li abundance in cool young stars. Our mean Li abundance, log N(Li) = 3.50 ± 0.07 is ～0.2 dex larger than the meteoritic value. While stellar models suggest that Li depletion of at least 0.4 dex, and possibly much larger, should have occurred in our lowest mass star(s), our Li abundances show no decline with decreasing mass indicative of such depletion.