(Proposal ID) S17B-042

(PI) Tsujimoto, Takuji

(Proposal Title)
The beginning of r-process enrichment in the Draco dwarf spheroidal galaxy

(Abstract)
Our two successful Subaru observations on the Draco dwarf spheroidal galaxy 
(dSph) have revealed the chemical feature of r-process elements for [Fe/H]>-2.5
in this galaxy. First we found a constant [Eu/H]  over the metallicity range of
-2<[Fe/H]<-1, implying no r-process events while more than 10^3 core-collapse 
supernovae increase the galactic Fe abundance. This level of rarity of r-
process production is compatible with the frequency expected for neutron star 
mergers. On the other hand, our latest observation discovered a sharp r-
process increase for [Fe/H]<-2; the stars with the metallicity range of -2.5<
[Fe/H]<-2 that we measured are clearly separated into two groups, which border 
around [Fe/H]~ -2.3 with the distinct difference by a nearly one order, 
suggesting the occurrence of a rare r-process event, the origin of which has 
not been identified. The last frontier of the Draco is [Fe/H]<-2.5. Since the 
Draco stars with [Fe/H]<-2 exhibit a pure r-process Ba/Eu ratio, the strong 
spectra of Ba become an accurate diagnosis of the r-process. We propose to 
measure Ba abundance for 3 stars with -3<[Fe/H]<-2.5 to aim at bridging the 
r-process path from the beginning to [Fe/H]=-2.5. Our project completed by 
this proposition  will yield the first galaxy which presents the whole 
evolutionary path of r-process and enables us to first assess the r-process 
producer mysteriously inclined to the early Universe.