Title: Abundance patterns of r-process enhanced metal-poor stars.

Author: S.Honda, W.Aoki, N.Christlieb, T.C.Beers, M.W.Hannawald,
T.Kajino, H.Ando, P.S.Barklem

Abstract:

We present detailed abundance measurements of neutron-capture elements
for very metal-poor stars having very large excesses of the r-process
elements.
After the Big Bang, the history of nucleogenesis is concerned mostly
with the physics of stellar evolution and nucleosynthesis in stars, and
with how the metal-enriched gas has mixed with the interstellar medium
from which subsequent stellar generations form.
The elements heavier than iron are produced by neutron-capture process.
There are two process, s- and r-process, which are generally thought to
occur stellar mass-loss or Super-Nova explosion.
However, the astrophysical site(s) for the r-process are still unclear.
An effective approach to understand this process from astronomical
observations is to measure the abundances of heavy metals in the
recently discovered very metal-poor stars enriched in r-process elements.
We obtained the three high-dispersion spectra of r-process-enhanced-stars
using Subaru/HDS, and determined the metallicity and abundances of
neutron-capture elements.
We obtained the following results.
1) We confirmed that those objects are very metal-poor ([Fe/H] < -2.5)
and show very large excess of neutron-capture elements relative to iron
(so-called r-II stars, which have [Eu/Fe] > +1.0).
These r-II stars exist only in a quite narrow metallicity range.
2) These objects are in agreement with the solar r-process pattern in
the range from the lighter to heavier (38 < Z < 70) neutron-capture element.
The effect of weak r-process is not seen.
3) The ratio of Th/Eu show scatter, but the width of scatter is
not so large.
This result will give a strong constraint on the modeling of the
r-process process.