(Proposal ID) S11B-030

(PI) Takada-Hidai, Masahide

(Proposal Title)
[S/Fe] Behavior of Very Metal-Poor Stars with the S I 1.046 mum Lines

(Abstract)
There is presently a serious and controversial dilemma concerning the behavior 
of [S/Fe] toward the very meal-poor regime ([Fe/H] < -2), which is of 
paramount importance to clarify the early-time history of Galactic 
nucleosynthesis; i.e., the weaker S i lines of RMT 6 at 8693-4 Angstrom 
suggest an increasing trend with decreasing [Fe/H], while the stronger S i 
lines of RMT 1 at 9212/9228/9237 Angstrom indicate a flat trend. With an 
intention to resolve this issue, we have newly observed in the S09A semester 
the S i lines of RMT 3 at 10455-9 Angstrom, which turned out to be superior 
to the so-far invoked two abundance indicators mentioned above. The [S/Fe] 
behavior resulting from our S09A observation shows a remarkably high value 
of ~ +0.7 - 0.8 at [Fe/H]~ -3 followed by a discontinuous jump between the 
narrow interval of -3.1 <[Fe/H]< -2.5. However, the number of observed stars 
in this [Fe/H] interval is not sufficiently large as to confirm that such 
behavior is real. To establish the [S/Fe] vs. [Fe/H] relation in [Fe/H] < -2, 
we propose the observations of the S i 1.046mum lines in zJ-band for 22 very 
metal-poor stars by using the IRCS+AO188 in the echelle spectroscopy mode with 
the resolution of R ~ 20000 and S/N ratio of > 200.