(Proposal ID) S14B-087

(PI) Tsujimoto Takuji

(Proposal Title)
 Searching for evidence of neutron star mergers as the origin of r-process element

(Abstract)
The origin of r-process elements remains unidentified and still puzzles us. 
Recent discovery of evidence for the ejection of r-process elements from a 
short-duration gamma-ray burst singled out neutron star (NS) mergers as their 
origin in addition to the most promising source of gravitational wave. Since 
each NS merger event is associated with the high yield of r-process elements, 
a significantly small event rate is required as compared to supernovae. Thus, 
if we can confirm or reject the rarity of r-process enrichment from stellar 
abundances, we will get a decisive answer for the origin of r-process. Dwarf 
spheroidal (dSph) galaxies in Local Group provide a direct test for this issue 
independent of any theoretical modeling since dSphs far less massive than the 
Galaxy are expected to host no events of NS merger over their whole past. This 
fact must be recorded as no increase in [Eu/H] as a function of time (or 
[Fe/H]) on stellar abundances if NS mergers are the major source of r-process. 
Here we propose to determine the [Eu/H] abundances of 6 red giants spanning a 
wide range of [Fe/H] in such a faint dSph, Draco. Draco is selected as our 
first target of this study since our results combined with previous Keck 
observation promise to draw a strong conclusion. No Eu evolution for Draco, 
if confirms, will give Galactic NS merger rate of <17 Myr^{-1}, leading to the 
detection rate of gravitational waves for KAGRA and others.