r-Process Abundances and Chronometers in Metal-poor Stars
r-Process Abundances and Chronometers in Metal-poor Stars
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DOI：
10.1086/307512
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发表时间：
1998-08
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影响因子：
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通讯作者：
J. Cowan;B. Pfeiffer;K. Kratz;F. Thielemann;C. Sneden;S. Burles;D. Tytler;T. Beers
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作者：
J. Cowan;B. Pfeiffer;K. Kratz;F. Thielemann;C. Sneden;S. Burles;D. Tytler;T. Beers
采用内部一致和物理上真实的核物理输入（准粒子随机相位近似[QRPA] β衰变特性和最近扩展的托马斯-费米与斯特鲁丁斯基积分和淬火（ETFSI-Q）核质量模型），进行了快速中子捕获（即r过程）核合成计算。这些理论计算假定（n,γ）的经典等待点近似为当下的（γ,n）平衡。计算重现了太阳同位素r丰度的细节，包括最重的稳定的Pb和Bi同位素。然后将这些计算结果与地面和哈勃太空望远镜对金属贫乏的晕星CS 22892-052、HD 115444、HD 122563和HD 126238中中子捕获元素的观测结果进行比较。四颗贫金属恒星的元素丰度与太阳r过程中元素Z≥56的元素分布一致。这些结果有力地表明，至少对于这些元素来说，相对元素r过程丰度在银河系的历史上没有改变。这也表明，太阳r过程丰度不太可能是来自不同r过程核合成位点的不同丰度模式的随机叠加。这进一步表明银河系中存在一个r-过程位点，至少对于Z≥56的元素来说是这样。利用观测到的恒星稳定元素丰度，结合太阳r过程丰度来约束计算，我们提出了放射性元素Th和u的零衰变年龄丰度的预测。我们将这些预测（通过质量模型ETFSI-Q获得，它能最好地再现太阳r丰度）与三颗金属非常缺乏的晕星（HD 115444， CS 22892-052和HD 122563）的新观测值进行比较。在观测误差范围内，CS 22892-052和HD 115444的[Th/Eu]比值相同。与理论比值比较表明，这两颗非常缺乏金属的恒星的平均年龄为15.6±4.6 Gyr，与早期的放射性年龄估计以及最近的球状和宇宙年龄估计一致。我们对HD 115444中铀丰度的上限也暗示了类似的年龄。这种对金属丰度非常低的恒星的放射性年龄测定避免了银河系化学演化模型中的不确定性。它们仍然包含不确定性，因为所涉及的核物理离β稳定性很远。然而，我们对预期的可能变化进行了广泛的概述，并得出结论，仅这一方面不应超过3 Gyr的限制。因此，这种方法有望作为一种独立的银河系测年技术。
Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (quasi-particle random-phase approximation [QRPA] β-decay properties and the recent extended Thomas-Fermi with Strutinsky integral and quenching (ETFSI-Q) nuclear mass model), have been performed. These theoretical computations assume the classical waiting-point approximation of (n,γ) ⇄ (γ,n) equilibrium. The calculations reproduce the solar isotopic r-abundances in detail, including the heaviest stable Pb and Bi isotopes. These calculations are then compared with ground-based and Hubble Space Telescope observations of neutron-capture elements in the metal-poor halo stars CS 22892-052, HD 115444, HD 122563, and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z ≥ 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This indicates also that it is unlikely that the solar r-process abundances resulted from a random superposition of varying abundance patterns from different r-process nucleosynthesis sites. This further suggests that there is one r-process site in the Galaxy, at least for elements Z ≥ 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, we present predictions for the zero decay-age abundances of the radioactive elements Th and U. We compare these predictions (obtained with the mass model ETFSI-Q, which reproduces solar r-abundances best) with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892-052, and HD 122563. Within the observational errors the ratio of [Th/Eu] is the same in both CS 22892-052 and HD 115444. Comparing with the theoretical ratio suggests an average age of these two very metal-poor stars to be 15.6 ± 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates. Our upper limit on the uranium abundance in HD 115444 also implies a similar age. Such radioactive age determinations of very low metallicity stars avoid uncertainties in Galactic chemical evolution models. They still include uncertainties due to the involved nuclear physics far from β-stability. However, we give an extensive overview of the possible variations expected and come to the conclusion that this aspect alone should not exceed limits of 3 Gyr. Therefore this method shows promise as an independent dating technique for the Galaxy.