The r-Process in the Neutrino Winds of Core-Collapse Supernovae and U-Th Cosmochronology
The r-Process in the Neutrino Winds of Core-Collapse Supernovae and U-Th Cosmochronology
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DOI：
10.1086/342230
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发表时间：
2002-06
期刊：
影响因子：
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通讯作者：
S. Wanajo;N. Itoh;Y. Ishimaru;S. Nozawa;T. Beers
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文献类型：
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作者：
S. Wanajo;N. Itoh;Y. Ishimaru;S. Nozawa;T. Beers
CS 31082-001（[Fe/H] = -2.9）是第二颗高r过程增强的贫金属星星，它的发现为年龄测定提供了一个强有力的新工具。由于238 U的半衰期是232 Th的三分之一，原则上，U-Th对可以提供比以前研究中使用的Th-Eu对更精确的宇宙计时器。在这个精密计时器的应用中，CS 31082-001的年龄可以被认为是银河系的最小年龄，因此也是宇宙的最小年龄。然而，这种方法的严重局限性之一是，U和Th的生产比的预测还没有在现实的天体物理模型的背景下的r-过程。我们有必要根据“中微子风”来建立这样一个模型，这种“中微子风”预计会从核心坍缩的超新星的新生中子星星中产生。在这个模型中，原中子星星的质量和（渐近）中微子球半径被假定为2.0兆和10公里，分别。最近的流体动力学研究表明，可能难以获得这样一个紧凑的（质量大和/或半径小）残留物。尽管如此，我们利用这组参数的选择，因为以前的工作表明，第三个r-过程峰值（因此U和Th）是很难达到时，采用一个不太紧凑的原中子星星的框架中的中微子风的情况。在中微子风稳定流动的假设下，考虑到广义相对论效应，得到了中子俘获过程中所涉及的材料的温度和密度的历史。电子分数被视为一个自由参数，随时间而定。在这些轨迹的r-过程核合成计算与核反应网络代码，包括锕系元素Z = 100。的质量积分r-过程的产量，通过假设一个简单的中微子光度的时间演变，得到的比较可用的光谱元素丰度数据CS 31082-001。因此，这颗星星的“年龄”被确定为14.1 ± 2.5 Gyr，与其他测年技术估计的宇宙年龄下限以及其他恒星放射性年龄估计非常一致。未来的测量Pt和Pb在这颗星星，以及扩大搜索额外的r-过程增强，金属贫星（特别是那些在U和Th都是可测量的），是特别重要的约束目前的天体物理模型的r-过程。
The discovery of the second highly r-process-enhanced, extremely metal poor star, CS 31082-001 ([Fe/H] = -2.9) has provided a powerful new tool for age determination by virtue of the detection and measurement of the radioactive species uranium and thorium. Because the half-life of 238U is one-third that of 232Th, the U-Th pair can, in principle, provide a far more precise cosmochronometer than the Th-Eu pair that has been used in previous investigations. In the application of this chronometer, the age of (the progenitor of) CS 31082-001 can be regarded as the minimum age of the Galaxy, and hence of the universe. One of the serious limitations of this approach, however, is that predictions of the production ratio of U and Th have not been made in the context of a realistic astrophysical model of the r-process. We have endeavored to produce such a model, based on the "neutrino winds" that are expected to arise from the nascent neutron star of a core-collapse supernova. In this model, the proto-neutron star mass and the (asymptotic) neutrino sphere radius are assumed to be 2.0 M☉ and 10 km, respectively. Recent hydrodynamic studies indicate that there may exist difficulties in obtaining such a compact (massive and/or small in radius) remnant. Nevertheless, we utilize this set of parameter choices since previous work suggests that the third r-process peak (and thus U and Th) is hardly reached when one adopts a less compact proto-neutron star in the framework of the neutrino-wind scenario. The temperature and density histories of the material involved in the neutron-capture processes are obtained with the assumption of a steady flow of the neutrino-powered winds, with general relativistic effects taken into account. The electron fraction is taken to be a free parameter, constant with time. The r-process nucleosynthesis in these trajectories is calculated with a nuclear reaction network code including actinides up to Z = 100. The mass-integrated r-process yields, obtained by assuming a simple time evolution of the neutrino luminosity, are compared to the available spectroscopic elemental abundance data of CS 31082-001. As a result, the "age" of this star is determined to be 14.1 ± 2.5 Gyr, in excellent agreement with lower limits on the age of the universe estimated by other dating techniques, as well as with other stellar radioactive age estimates. Future measurements of Pt and Pb in this star, as well as expansion of searches for additional r-process-enhanced, metal-poor stars (especially those in which both U and Th are measurable), are of special importance to constrain the current astrophysical models for the r-process.