Chemical Feature of Eu abundance in the Draco dwarf spheroidal galaxy
  Chemical abundance of r-process elements in nearby dwarf spheroidal (dSph)
galaxies is a powerful tool to probe the site of r-process since their
small-mass scale can sort out individual events producing r-process elements. A
merger of binary neutron stars is a promising candidate of this site. In faint,
or less massive dSph galaxies such as the Draco, a few binary neutron star
mergers are expected to have occurred at most over the whole past. We have
measured chemical abundances including Eu and Ba of three red giants in the
Draco dSph by Subaru/HDS observation. The Eu detection for one star with
[Fe/H]=-1.45 confirms a broadly constant [Eu/H] of ~-1.3 for stars with
[Fe/H]>-2. This feature is shared by other dSphs with similar masses, i.e., the
Sculptor and the Carina, and suggests that neutron star merger is the origin of
r-process elements in terms of its rarity. In addition, two very metal-poor
stars with [Fe/H]=-2.12 and -2.51 are found to exhibit very low Eu abundances
such as [Eu/H]<-2 with an implication of a sudden increase of Eu abundance by
more than 0.7 dex at [Fe/H] ~ -2.2 in the Draco dSph. In addition, the
detection of Ba abundances for these stars suggests that the r-process
enrichment initiated no later than the time when only a few % of stars in the
present-day Draco dSph was formed. Though an identification of the origin of an
early Eu production inside the Draco dSph should be awaited until more
abundance data of stars with [Fe/H]<-2 in the Draco as well as other faint
dSphs become available, the implied early emergence of Eu production event
might be reconciled with the presence of extremely metal-poor stars enriched by
r-process elements in the Galactic halo.
