The rate of the triple-α reaction that forms 12C affects1,2 the synthesis of heavy elements in the Ga-Cd range in proton-rich neutrino-driven outflows of core-collapse supernovae3-5. Initially, these outflows contain only protons and neutrons; these later combine to form α particles, then 12C nuclei via the triple-α reaction, and eventually heavier nuclei as the material expands and cools. Previous experimental work6,7 demonstrated that despite the high temperatures encountered in these environments, the reaction is dominated by the well characterized Hoyle state resonance in 12C nuclei. At sufficiently high nucleon densities, however, proton- and neutron-scattering processes may alter the effective width of the Hoyle state8,9. 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-α 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 νp process3-5 (where ν is the neutrino and p is the proton) in the innermost ejected material of supernovae10-13. Previous work on the rate enhancement mechanism9 did not anticipate the importance of this enhancement for proton-rich nucleosynthesis. Because the in-medium contribution to the triple-α 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 factor1,2, because the enhancement depends on the evolving thermodynamic conditions. The resulting suppression of heavy-element nucleosynthesis for realistic conditions casts doubt on the νp process being the explanation for the anomalously high abundances of 92,94Mo and 96,98Ru isotopes in the Solar System1,3,14 and for the signatures of early Universe element synthesis in the Ga-Cd range found in the spectra of ancient metal-poor stars15-20.

中文翻译：

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增强的三α反应减少了超新星中富含质子的核合成

形成 12C 的三重 α 反应的速率影响 1,2 核心坍缩超新星 3-5 的富含质子的中微子驱动外流中 Ga-Cd 范围内重元素的合成。最初，这些流出物仅包含质子和中子；这些后来结合形成α粒子，然后通过三重α反应形成12C核，最终随着材料膨胀和冷却形成更重的核。先前的实验工作 6, 7 表明，尽管在这些环境中遇到高温，但反应以 12C 原子核中充分表征的霍伊尔态共振为主。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。这就提出了超新星外流的反应速率是多少的问题，以及变化如何影响核合成预测。在这里，我们报告说，在富含质子的核心坍缩超新星外流中，这些迄今为止被忽视的过程将三重α反应速率提高了一个数量级。较大的反应速率抑制了由 νp 过程 3-5（其中 ν 是中微子，p 是质子）在超新星 10-13 的最内部喷射材料中形成的重质子富同位素的产生。先前关于速率增强机制的工作没有预料到这种增强对富含质子的核合成的重要性。由于介质对三α反应速率的贡献必须以高密度存在，因此需要将这种效应包含在超新星核合成模型中。这种增强也不同于早期的敏感性研究，后者通过常数因子 1,2 探索未增强速率的变化，因为增强取决于不断发展的热力学条件。在现实条件下对重元素核合成的抑制使人们怀疑 νp 过程是否解释了太阳系中 92,94Mo 和 96,98Ru 同位素的异常高丰度 1,3,14 以及早期宇宙元素的特征在古代贫金属恒星光谱中发现的 Ga-Cd 范围内的合成 15-20。