Enrichment in r-process elements from multiple distinct events in the
  early Draco dwarf spheroidal galaxy
  The stellar record of elemental abundances in satellite galaxies is important
to identify the origin of r-process because such a small stellar system could
have hosted a single r-process event, which would distinguish member stars that
are formed before and after the event through the evidence of a considerable
difference in the abundances of r-process elements, as found in the ultra-faint
dwarf galaxy Reticulum II (Ret II). However, the limited mass of these systems
prevents us from collecting information from a sufficient number of stars in
individual satellites. Hence, it remains unclear whether the discovery of a
remarkable r-process enrichment event in Ret II explains the nature of
r-process abundances or is an exception. We perform high-resolution
spectroscopic measurements of r-process abundances for twelve metal-poor stars
in the Draco dwarf galaxy in the metallicity range of -2.5<[Fe/H]<-2. We found
that these stars are separated into two groups with r-process abundances
differing by one order of magnitude. A group of stars with high abundances of
r-process elements was formed by a single r-process event that corresponds to
the event evidenced in Ret II. On the other hand, the low r-process abundance
group was formed by another sporadic enrichment channel producing a far fewer
r-process elements, which is clearly identified for the first time.
Accordingly, we identified two populations of stars with different r-process
abundances, which are built by two r-process events that enriched gases at
levels that differ by more than one order of magnitude.
