We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from HST/STIS, Keck/HIRES, and VLT/UVES spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the 3rd r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical "waiting-point" approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 < log(n_n) < 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the "main" r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect--at most--only the elements beyond the 3rd r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance "losses" by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.
Nucleosynthesis in Rotating massive stars and Abundances in the Early Galaxy
我们讨论了在低金属丰度的轴向旋转的三个影响。第一个是化学物种的混合，预计在低金属量环境中更有效。结果是产生大量的初级$^{14}$ N、$^{13}$ C、$^{22}$ Ne，并对{ \it s}-过程元素的核合成产生强烈影响。第二个效应是第一个效应的结果。强烈的混合使得大量的CNO元素有可能出现在表面。这增加了外层的不透明度，并可能引发线驱动风的重要质量损失。第三个效应是，在主序星阶段，金属丰度非常低的恒星比金属丰度更高的恒星更容易达到临界速度\footnote{The critical velocity is the surface equatorial velocity such that the centrifugal acceleration compensates for the local gravity.}。我们讨论了这三种效应作为金属丰度的函数的各自的重要性。我们显示的后果早期化学演化的星系晕和解释CEMP星。我们的结论是，旋转可能是一个关键的功能，有助于在一个重要的方式来塑造宇宙中的第一代恒星的演化。
