(Proposal ID) S11A-024

(PI) Aoki, Wako

(Proposal Title)
U/Th chronometry for a red giant in the dwarf spheroidal galaxy Ursa Minor

(Abstract)
We apply the U/Th chronometry to a red giant, COS82, in the dwarf spheroidal 
galaxy Ursa Minor by the measurements of Th and U abundances based on high 
resolution spectroscopy with HDS. Our previous studies with HDS revealed that 
this object with [Fe/H]= -1.5 shows extremely large enhancements of heavy 
neutron-capture elements, and their abundance pattern almost completely agrees 
with the r-process abundance pattern in the solar system, indicating the 
detectability of the heaviest nuclei U and Th is highest among the stars known 
to date. While one Th line is already detected in our spectrum obtained in a 
previous run, our spectrum synthesis shows that one U line and several Th lines
 are detectable in a higher quality, blue spectrum. The purpose of this 
measurement is to provide a new, independent calibration of the age estimates 
of red giants in the Ursa Minor dwarf galaxy, which are not well constrained 
from a color magnitude diagram (CMD). This will provide a unique opportunity 
to distinguish two possibilities that red giants of this galaxy are very old 
in general, as expected from CMD for turn-off stars, or some of them are 
relatively young, as suggested by some chemical evolution models that explain 
low alpha/Fe ratios found in red giants in this galaxy.