(Proposal ID) S15B-078

(PI) Tsujimoto, Takuji

(Proposal Title)
Unraveling the Ba feature of the least massive galaxy Segue 2

(Abstract)
By our previous successful Subaru observation, we have confirmed Eu abundance 
stays constant for a significantly long period of star formation in classical 
dwarf spheroidal (dSph) galaxies, and revealed r-process elements are of the 
neutron star (NS) merger origin, discarding the supernova-origin-scenario. 
The same conclusion can be deduced from the Ba abundance feature in ultra-
faint dSphs since these galaxies are so unevolved that s-process is not 
operated. In contrast, classical dSphs exhibit an increasing Ba trend as a 
result of enrichment by s-process, different from their constant Eu. 
This suggests that Ba abundance at the metal-rich end is quite different 
between classical and ultra-faint dSphs. Segue 2 is the least massive galaxy 
in the Local Group, and its chemistry/origin is a mystery. In spite of the 
least mass, its mean metallicity is equivalent to classical dSphs, deviating 
from the mass-metallicity relation. This fact leads to the implication that 
Segue 2 is a remnant of a tidally disrupted classical dSph. If not, Segue 2 is 
originally the least massive and can be the first candidate of a survivor of 
building blocks of Galactic halo. The two possibilities can be judged by high 
or low Ba abundance. To assess the veiled origin of Segue 2, we propose to 
measure Ba abundances of 2-4 stars at [Fe/H]~ -2.1�}0.2 where [Ba/H] is 
expected to {differ by ~2 dex} between classical and ultra-faint dSphs.