Spectroscopic Studies of Extremely Metal‐Poor Stars with the Subaru High Dispersion Spectrograph. II. Ther‐Process Elements, Including Thorium

We present the abundance analyses for the neutron-capture elements, anddiscuss the observed abundance distributions in very metal-poor stars withexcesses of r-process elements. As has been found by previous abundancestudies, the star-to-star scatter in the abundances of neutron-capture elementsare 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 theneutron-capture elements are similar to that of the solar system r-processcomponent. These results strongly suggest that the heavy neutron-captureelements in these objects are primarily synthesized by the r-process. On theother hand, the abundance ratios of the light neutron-capture elements (38$\leq$ Z $\leq$ 46) exhibit a rather large dispersion. Our inspection of thecorrelation between Sr and Ba abundances in very metal-poor stars reveals thatthe dispersion of the Sr abundances clearly decreases with increasing Baabundance. This results support previous suggestions that the lightneutron-capture elements are likely to have been produced in differentastrophysical sites from those associated with the production of the heavierones. The Th/Eu abundance ratios (log(Th/Eu)) measured for the sevenr-process-enhanced stars range from -0.10 to -0.59. Since these very metal-poorstars are believed to be formed in the early Galaxy, this result means that asmall dispersion appears in the abundance ratios between Th and rare-earthelements, such as Eu, in very metal-poor stars. In order to apply the Th/Euratios to estimates of stellar ages, further understanding for the Thproduction by the r-process nucleosynthesis is required.Comment: 53 pages, 26 Postscript figures, accepted for publication in Ap