Spectroscopic Studies of Extremly Metal-Poor Stars with Subaru/HDS:II.The r-process Elements, Including Thorium

Details Spectroscopic Studies of Extremly Metal-Poor Stars with Subaru/HDS:II.The r-process Elements, Including Thorium Spectroscopic Studies of Extremly Metal-Poor Stars with Subaru/HDS:II.The r-process Elements, Including Thorium

2004-02-12

arxiv.org/abs/astro-ph/0402298v1

Astrophys.J. 607 (2004) 460-473

Astronomy and Astrophysics

Astrophysics

53 pages, 26 Postscript figures, accepted for publication in ApJ

Scientific paper

10.1086/383406

We present the abundance analyses for the neutron-capture elements, and discuss the observed abundance distributions in very metal-poor stars with excesses of r-process elements. As has been found by previous abundance studies, the star-to-star scatter in the abundances of neutron-capture elements are very large. The abundance patterns of the heavy neutron-capture elements (56 $\leq$ Z $\leq$ 70) in seven objects with moderate to large excesses of the neutron-capture elements are similar to that of the solar system r-process component. These results strongly suggest that the heavy neutron-capture elements in these objects are primarily synthesized by the r-process. On the other hand, the abundance ratios of the light neutron-capture elements (38 $\leq$ Z $\leq$ 46) exhibit a rather large dispersion. Our inspection of the correlation between Sr and Ba abundances in very metal-poor stars reveals that the dispersion of the Sr abundances clearly decreases with increasing Ba abundance. This results support previous suggestions that the light neutron-capture elements are likely to have been produced in different astrophysical sites from those associated with the production of the heavier ones. The Th/Eu abundance ratios (log(Th/Eu)) measured for the seven r-process-enhanced stars range from -0.10 to -0.59. Since these very metal-poor stars are believed to be formed in the early Galaxy, this result means that a small dispersion appears in the abundance ratios between Th and rare-earth elements, such as Eu, in very metal-poor stars. In order to apply the Th/Eu ratios to estimates of stellar ages, further understanding for the Th production by the r-process nucleosynthesis is required.

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