Speaker
Wako Aoki
(National Astronomical Observatory of Japan)
Description
High resolution spectroscopy for very metal-poor stars have revealed
that some fraction of objects have large excesses of neutron-capture
elements, whose abundance patterns agree very well with that of the
r-process component of solar-system material (e.g. Sneden et al. 1996,
ApJ, 467, 819). However, recent abundance studies show the existence
of objects that have quite different abundance patterns.
One is the class of objects that have very large enhancement of light
neutron-capture elements. Although the nucleosynthesis process
responsible for such chemical composition is still unknown,
observational studies for this process have made substantial
progresses in the past few years: (1) Such objects appear even in the
extremely low metallicity range ([Fe/H]~<-3.0), while the stars having
large excesses of heavy neutron-capture elements appear in the
metallicity range of [Fe/H]>~-3.0. (Aoki et al. 2005, ApJ 632,
611). A large excess of the light neutron-capture element Sr is found
even in the most iron-deficient star HE1327-2326 (Frebel et al. 2005,
Nature 434, 871). (2) An evidence of this process is found in
metal-poor globular cluster stars (Otuski et al. 2006, ApJL, in
press). These observational facts indicate that the process was
efficient in general in the very early Galaxy. (3) The detailed
elemental abundance pattern (Sr-Yb) was determined for the metal-poor
star HD122563, a star that might well preserve the yields of this
process (Honda et al. 2006, ApJ, in press). The abundances of elements
between the 1st and 2nd abundance peaks of neutron-capture elements
continuously decrease in this object. This result provides strong
constraints on modeling the process responsible for production of
light neutron-capture elements in the early Galaxy, which is
presumably related to early generation supernovae.
Another is the class of carbon-enhanced metal-poor stars that exhibit
large excesses of Eu as well as s-process elements (e.g. Ba,
Pb). Although contributions of nucleosynthesis in AGB stars are
assumed because of the excesses of s-process elements, standard
s-process models cannot explain the Eu enhancement. Our recent study
determined the abundances of Os and Ir for one star in this group
(CS31062-050), confirming the excesses of r-process
elements. Discovery of such objects and measurements of their detailed
abundance patterns have impact on the studies of the origins of
r-process elements.
Author
Wako Aoki
(National Astronomical Observatory of Japan)
