Chemical abundance analysis of extremely metal-poor stars in the Sextans
  dwarf spheroidal galaxy
  Metal-poor components of dwarf galaxies around the Milky Way could be
remnants of the building blocks of the Galactic halo structure. Low-mass stars
that are currently observed as metal-poor stars are expected to have formed in
chemically homogeneous clusters in the early phases of galaxy formation. They
should have already disintegrated and should exhibit large scatter in abundance
ratios of some sets of elements (e.g., Sr/Ba) in the Milky Way field stars.
However, chemical abundance ratios are expected to cluster in very metal-poor
stars in dwarf galaxies because the number of clusters formed in individual
galaxies in the very early phase is expected to be quite limited. We examine
the possible clustering of abundance ratios of Sr and Ba in the Sextans dwarf
galaxy to test for the clustering star formation scenario. We investigate a
total of 11 elements (C, Mg, Ca, Sc, Ti, Cr, Mn, Ni, Zn, Sr, Ba) in five stars
in the Sextans dwarf galaxy. Previous studies suggest that these have similar
abundance ratios. In this study, we focus on the abundance ratio of Sr to Ba.
The observations are based on high-resolution spectroscopy (R = 40 000) using
the Subaru Telescope High Dispersion Spectrograph. The distribution of
$\alpha$/Fe abundance ratios of the Sextans dwarf galaxy stars is slightly
lower than the average of the values of stars in the Galactic halo. The Sr/Ba
abundance ratios for the five metal-poor stars are in good agreement, and this
clumping is distinctive compared to the [Sr/Ba] spread seen in the metal-poor
halo stars. We find that the probability of such clumping is very small if the
Sextans stars have distributions of Sr and Ba abundances similar to halo stars.
