(Proposal ID) S11A-129

(PI) Ito, Hiroko

(Proposal Title)
Lithium abundances of the most metal-poor turnoff stars

(Abstract)
In order to solve the lithium problem, the discrepancy between the observed Li 
abundance plateau of metal-poor turnoff stars and the primordial Li value 
predicted by the CMB observations and standard Big Bang nucleosynthesis models,
 the possibility of stellar intrinsic Li depletion needs to be first examined. 
The scatter of Li abundances recently found for extremely metal-poor ([Fe/H] < 
-3.0) turnoff stars opens a possibility to investigate the correlation between 
the Li depletion and stellar properties, such as effective temperature and 
metallicity, which will give crucial constraints on depletion model 
predictions. Especially, intensive Li abundance study at the lowest metallicity
 ([Fe/H]<-3.5) is essential, because the largest Li depletion seems to appear 
in this metallicity range. We propose here to obtain high-resolution and 
high-S/N spectra of seven candidates of the most metal-poor turnoff stars 
having [Fe/H] < -3.5 with Subaru/HDS. This program, combined with the data of 
our previous runs, will increase the number of Li abundance measurements in 
[Fe/H] < -3.5 from 2 to more than 12, and allow us to discuss the dependence 
of Li abundances on stellar properties, which leads to better understanding of 
Li depletion processes, and (at least partial) solution of the lithium problem.