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

We have obtained high-resolution, high signal-to-noise near-UV-blue spectra of 22 very metal-poor stars ([Fe/H] < -2.5) with the Subaru High Dispersion Spectrograph and measured the abundances of elements from C to Th. The metallicity range of the observed stars is -3.2 < [Fe/H] < -2.4. As found by previous studies, the star-to-star scatter in the measured abundances of neutron-capture elements in these stars is very large, much greater than could be assigned to observational errors, in comparison with the relatively small scatter in the α- and iron-peak elements. In spite of the large scatter in the ratios of the neutron-capture elements relative to iron, the abundance patterns of heavy neutron-capture elements (56 ≤ Z ≲ 72) are quite similar within our sample stars. The Ba/Eu ratios in the 11 very metal-poor stars in our sample in which both elements have been detected are nearly equal to that of the solar system r-process component. Moreover, the abundance patterns of the heavy neutron-capture elements (56 ≤ Z ≤ 70) in seven objects with clear enhancements of the neutron-capture elements are similar to that of the solar system r-process component. These results prove that heavy neutron-capture elements in these objects are primarily synthesized by the r-process. In contrast, the abundance ratios of the light neutron-capture elements (38 ≤ Z ≤ 46) relative to the heavier ones (56 ≤ Z ≤ 70) exhibit a 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 trend is naturally explained by hypothesizing the existence of two processes, one that produces Sr without Ba and another that produces Sr and Ba in similar proportions. This result should provide a strong constraint on the origin of the light neutron-capture elements at low metallicity. We have identified a new highly r-process element enhanced, metal-poor star, CS 22183-031, a giant with [Fe/H] = -2.93 and [Eu/Fe] = +1.2. We also identified a new, moderately r-process-enhanced, metal-poor star, CS 30306-132, a giant with [Fe/H] = -2.42 and [Eu/Fe] = +0.85. The abundance ratio of the radioactive element Th (Z = 90) relative to the stable rare-earth elements (e.g., Eu) in very metal-poor stars has been used as a cosmochronometer by a number of previous authors. Thorium is detected in seven stars in our sample, including four objects for which the detection of Th has already been reported. New detections of thorium have been made for the stars HD 6268, HD 110184, and CS 30306-132. The Th/Eu abundance ratios [log(Th/Eu)], are distributed over the range -0.10 to -0.59, with typical errors of 0.10 to 0.15 dex. In particular, the ratios in two stars, CS 31082-001 and CS 30306-132, are significantly higher than the ratio in the well-studied object CS 22892-052 and those of other moderately r-process-enhanced metal-poor stars previously reported. Since these very metal-poor stars are believed to be formed in the early Galaxy, this result suggests that the abundance ratios between Th and stable rare-earth elements such as Eu, both of which are presumably produced by r-process nucleosynthesis, may exhibit real star-to-star scatter, with implications for (1) the astrophysical sites of the r-process, and (2) the use of Th/Eu as a cosmochronometer.