Silicon and Strontium abundances of very metal-poor stars determined
  from near-infrared spectra
  Silicon and Strontium are key elements to explore the nucleosynthesis and
chemical evolution of the Galaxy by measurements of very metal-poor stars.
There are, however, only a few useful spectral lines of these elements in the
optical range that are measurable for such low-metallicity stars. Here we
report on abundances of these two elements determined from near-infrared
high-resolution spectra obtained with the Subaru Telescope Infrared Doppler
instrument (IRD). Si abundances are determined for as many as 26 Si lines for
six very and extremely metal-poor stars (-4.0<[Fe/H]<-1.5), which significantly
improves the reliability of the abundance measurements. All six stars,
including three carbon-enhanced objects, show over-abundances of Si
([Si/Fe]~+0.5). Two stars with [Fe/H]~-1.5 have relatively small
over-abundances. The [Mg/Si] ratios agree with the solar value, except for one
metal-poor star with carbon excess. Strontium abundances are determined from
the triplet lines for four stars, including two for the first time. The
consistency of the Sr abundances determined from near-infrared and optical
spectra require further examination from additional observations.
