(Proposal ID) S16A-097

(PI) Tsujimoto, Takuji

(Proposal Title)
Identifying the r-process site of the Draco dSph galaxy

(Abstract)
Our previous Subaru observations combined with our theoretical studies have 
found compelling evidence for neutron star (NS) mergers as the r-process 
origin in the observed chemical feature of Eu in the Draco, Sculptor and 
Carina dSphs; these dSphs exhibit a constant [Eu/H] of ~-1.3 over the 
metallicity range of -2<[Fe/H]<-1, implying no Eu production events while 
thousands of core-collapse supernovae increasing the galactic Fe abundance. 
This level of rarity of Eu production is compatible with the frequency 
expected for NS mergers. Then, we confront the mystery-How do these dSphs 
which do not undergo any Eu production events for [Fe/H]>-2 are enriched 
up to [Eu/H]~ -1.3 until [Fe/H]~ -2? A small data set for the present imply 
that [Eu/H] remarkably increases inside the early dSphs but the evolutionary 
path remains veiled. Since dSph stars with [Fe/H]<-2 exhibit a pure r-process 
Ba/Eu ratio, their Ba abundances can trace the r-process enrichment instead of 
(mostly the upper limits of ) Eu abundances. Thus far we have [Ba/H] data for 
only six stars for [Fe/H]<-2 in the Draco. We propose to measure Ba abundance 
for 12 stars with [Fe/H]<-2, some of which were newly identified as the 
members of the Draco. This large increment of data promise to reveal the 
evolutionary path of r-process elements and convincingly assess the r-process 
producer in the early dSphs.