(Proposal ID) S15A-069

(PI) Tsujimoto, Takuji

(Proposal Title)
 First Eu detection in Sextans to test neutron star mergers as the r-process site

(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. Highlighting the rarity of NS merger events as compared to supernovae, 
we have been proposing a new test on this issue using faint dwarf spheroidal 
(dSph) galaxies in the Local Group. This test is to see whether the Eu 
abundance on stellar surfaces stays constant as a function of time (or [Fe/H]),
 possibly associated with a sudden increase of Eu abundance due to  a single 
NS merger event providing a large amount of Eu in some faint dSphs. A positive 
result, if obtained, will certainly lead us to the conclusion that r-process 
elements are of the NS merger origin because faint dSphs far less massive than 
the Galaxy are expected to host few events of NS mergers over their whole past.
Among only five dSphs suitable to this test, the feature of Eu abundance in 
the Sextans dSph has been thus far completely veiled. Here we propose to 
measure six sufficiently bright red giants to obtain useful information on the
 Eu abundances in the wide range of [Fe/H]   by Subaru HDS observations. 
Thus our proposition will acquire the utmost data critical to identifying 
the r-process origin from a new entry of the Sextans dSph.