Enhanced triple-α reaction reduces proton-rich nucleosynthesis in supernovae
Enhanced triple-α reaction reduces proton-rich nucleosynthesis in supernovae
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DOI：
10.1038/s41586-020-2948-7
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发表时间：
2020-12-03
期刊：
影响因子：
64.8
通讯作者：
Schatz, Hendrik
中科院分区：
文献类型：
--
作者：
Jin, Shilun;Roberts, Luke F.;Schatz, Hendrik
在高核子密度下，核心坍缩超新星中的三α反应速率会增强，从而抑制了从镓到镉的富质子核的形成。形成C-1,2的三α反应速率影响（1，2）核心坍缩超新星中的富质子中微子驱动流出物中Ga-Cd范围内重元素的合成（3-5）。最初，这些流出物只包含质子和中子;这些物质后来联合收割机结合形成α粒子，然后通过三α反应形成C-12核，最终随着材料膨胀和冷却形成更重的核。以前的实验工作（6，7）表明，尽管在这些环境中遇到高温，反应是由C-12核中的霍伊尔态共振所主导的。然而，在足够高的核子密度下，质子和中子散射过程可能会改变霍伊尔态的有效宽度（8，9）。这就提出了超新星流出的反应速率是多少，以及变化如何影响核合成预测的问题。在这里，我们报告说，在富含质子的核心坍缩超新星流出，这些迄今被忽视的过程提高了三阿尔法反应速率高达一个数量级。较大的反应速率抑制了超新星（10-13）最内部喷射物质中由nu p过程（3-5）（其中nu是中微子，p是质子）形成的重质子富同位素的产生。以前关于速率增强机制的工作（9）没有预料到这种增强对富质子核合成的重要性。由于介质对三α反应速率的贡献必须在高密度下存在，因此需要将这种效应纳入超新星核合成模型。这种增强也不同于早期的敏感性研究，该研究探索了未增强速率的恒定因子（1，2）的变化，因为增强取决于不断变化的热力学条件。在现实条件下，重元素核合成的抑制结果使人们对nup过程能否解释太阳系中Mo-92、Mo-94和Ru-96、Ru-98同位素的异常高丰度（1、3、14）以及在古老贫金属恒星光谱中发现的Ga-Cd范围内早期宇宙元素合成的特征（15-20）产生了怀疑。
The triple-alpha reaction rate in proton-rich core-collapse supernovae is found to be enhanced at high nucleon densities, suppressing the formation of proton-rich nuclei from gallium to cadmium.The rate of the triple-alpha reaction that forms C-12 affects(1,2) the synthesis of heavy elements in the Ga-Cd range in proton-rich neutrino-driven outflows of core-collapse supernovae(3-5). Initially, these outflows contain only protons and neutrons; these later combine to form alpha particles, then C-12 nuclei via the triple-alpha reaction, and eventually heavier nuclei as the material expands and cools. Previous experimental work(6,7) demonstrated that despite the high temperatures encountered in these environments, the reaction is dominated by the well characterized Hoyle state resonance in C-12 nuclei. At sufficiently high nucleon densities, however, proton- and neutron-scattering processes may alter the effective width of the Hoyle state(8,9). This raises the questions of what the reaction rate in supernova outflows is, and how changes affect nucleosynthesis predictions. Here we report that in proton-rich core-collapse supernova outflows, these hitherto neglected processes enhance the triple-alpha reaction rate by up to an order of magnitude. The larger reaction rate suppresses the production of heavy proton-rich isotopes that are formed by the nu p process(3-5) (where nu is the neutrino and p is the proton) in the innermost ejected material of supernovae(10-13). Previous work on the rate enhancement mechanism(9) did not anticipate the importance of this enhancement for proton-rich nucleosynthesis. Because the in-medium contribution to the triple-alpha reaction rate must be present at high densities, this effect needs to be included in supernova nucleosynthesis models. This enhancement also differs from earlier sensitivity studies that explored variations of the unenhanced rate by a constant factor(1,2), because the enhancement depends on the evolving thermodynamic conditions. The resulting suppression of heavy-element nucleosynthesis for realistic conditions casts doubt on the nu p process being the explanation for the anomalously high abundances of Mo-92,Mo-94 and Ru-96,Ru-98 isotopes in the Solar System(1,3,14) and for the signatures of early Universe element synthesis in the Ga-Cd range found in the spectra of ancient metal-poor stars(15-20).