r-process Enrichment in the Galactic Halo Characterized by Nucleosynthesis Variation in the Ejecta of Coalescing Neutron Star Binaries
r-process Enrichment in the Galactic Halo Characterized by Nucleosynthesis Variation in the Ejecta of Coalescing Neutron Star Binaries
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DOI：
10.3847/1538-4357/ab655c
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发表时间：
2019-12
期刊：
影响因子：
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通讯作者：
T. Tsujimoto;N. Nishimura;K. Kyutoku
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文献类型：
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作者：
T. Tsujimoto;N. Nishimura;K. Kyutoku
从银晕恒星的[Eu/Fe]比率可以看出，r过程元素的丰度在恒星之间有很大的变化，这是一个与其他重元素不同的显著特征。这在一定程度上是由于存在高度富含r过程的恒星，被归类为r-II星([Eu/Fe]≥+1)。平行地，晕星表明轻的r过程元素(Y)与Eu的比率与[Eu/Fe]密切相关，给出了在r-II星稳定的最低的[Y/Eu]比率。另一方面，最近对合并双中子星(CNSNSS)的流体力学模拟表明，r过程位置可以分为两类，提供不同的电子分数分布：潮汐驱动的动力抛射和(动力的或合并后的)非潮汐抛射。在这里，我们表明，[Eu/Fe]的广泛跨越特征可以通过考虑来自cNSNS和聚合黑洞/中子星双星(CBHNSS)的潮汐驱动动力抛射的不同质量的模型来再现。此外，观察到的[Y/Eu]趋势可以用双NS系统中两种质量不对称的抛射物的联合核合成来解释。我们的设想表明，巨大的潮汐驱动的动力喷发伴随着cNSNSS或cBHNSS的大量非潮汐部分，可以单独容纳r-II丰度，在某些情况下还包括一种鳗系元素的提升。我们的研究估计的cNSNSS的事故率与引力波探测的∼1000(90%可信区间为110-3840)GPC−3yr−1的最新结果一致，并预测每年每GPC 3会出现几个与r过程生产相关的cBHNSS事件。
A large star-to-star variation in the abundances of r-process elements, as seen in the [Eu/Fe] ratio for Galactic halo stars, is a prominent feature that is distinguishable from other heavy elements. It is, in part, caused by the presence of highly r-process-enriched stars, classified as r-II stars ([Eu/Fe] ≥ + 1). In parallel, halo stars show that the ratio of a light r-process element (Y) to Eu is tightly correlated with [Eu/Fe], giving the lowest [Y/Eu] ratio that levels off at r-II stars. On the other hand, recent hydrodynamical simulations of coalescing double neutron stars (cNSNSs) have suggested that r-process sites may be separated into two classes providing different electron-fraction distributions: tidally driven dynamical ejecta and (dynamical or postmerger) nontidal ejecta. Here, we show that a widely spanning feature of [Eu/Fe] can be reproduced by models that consider the different masses of tidally driven dynamical ejecta from both cNSNSs and coalescing black hole/neutron star binaries (cBHNSs). In addition, the observed [Y/Eu] trend is explained by the combined nucleosynthesis in two kinds of ejecta with varying mass asymmetry in double NS systems. Our scenario suggests that massive tidally driven dynamical ejecta accompanied by massive nontidal part from cNSNSs or cBHNSs could alone accommodate r-II abundances, including an actinide boost in some cases. The event rate for cNSNSs estimated from our study agrees with the latest result of ∼1000 (90% confidence interval of 110–3840) Gpc−3yr−1 by gravitational-wave detection, and a few events per Gpc3 per year of cBHNSs associated with r-process production are predicted to emerge.