(Proposal ID) S11B-035

(PI) Aoki, Wako

(Proposal Title)
Ba isotope ratios in most metal-poor stars

(Abstract)
We propose to measure Ba isotope ratios in most metal-poor stars ([Fe/H]~ -4) 
as a probe of the origins of heavy neutron-capture elements and the nature of 
the r-process. In stars even with such low metallicity some amount of heavy 
elements are contained, though they are usually under-abundant (e.g. 
[Ba/H]~ -5). If their source is the r-process, as usually expected for very 
metal-poor stars, the r-process should be a common phenomenon in core-collapse 
supernovae. This is an important constraint on the nature and the astrophysical
 sites of the r-process which are not well understood yet. However, there is no
 strong observational evidence for the assumption that the heavy elements in 
such low metallicity objects were synthesized by the r-process. Ba isotope 
ratios (fraction of the odd nuclei ^135,137Ba) are useful to distinguish their 
origin (r-process or s-process). If the ratios do not agree with the value 
expected for the r-process, but are consistent with the s-process one (or 
solar-system value), accretion of interstellar matter should be revisited as 
the source of heavy elements in most metal-poor stars. The Ba isotope ratios 
will be measured for two red giants with [Fe/H]~ -4 from the profile analysis 
of the Ba II resonance lines obtained with HDS and its new image slicer.