Signatures of r-process Elements in Kilonova Spectra
Signatures of r-process Elements in Kilonova Spectra
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DOI：
10.3847/1538-4357/abf358
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发表时间：
2020-06
期刊：
影响因子：
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通讯作者：
N. Domoto;Masaomi Tanaka;S. Wanajo;K. Kawaguchi
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文献类型：
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作者：
N. Domoto;Masaomi Tanaka;S. Wanajo;K. Kawaguchi
双中子星(NS)的合并有望合成r过程元素，并发出被称为kironovae的放射性能量辐射。虽然GW170817/AT2017gfo的观测证实了r-过程核合成，但除了锶之外，没有发现个别元素的踪迹。在这篇文章中，我们系统地计算了束缚-束缚跃迁的谱线强度，并对NS合并抛射的辐射传输进行了模拟，用于千诺瓦谱中的元素识别。我们发现，在贫镧模型的光谱中出现了SrII三重线，这与GW170817/AT2017gfo的吸收特征是一致的。合成光谱还显示出较强的CaII三重线。这是很自然的，因为钙和锶是在电子含量相对较高的材料中共同产生的，而且它们的离子具有相似的原子结构，只有一个S电子在最外层。然而，谱线强度在很大程度上取决于喷出物中的丰度分布和温度。对于我们的富镧系元素模型，光谱显示了Ce、Tb和Th等双电离重元素的特征。我们的结果表明，GW170817/AT2017gfo的成线区是贫镧系元素。结果表明，SrII和CaII谱线可以作为NS合并喷发过程中物理条件的探测。GW170817/AT2017gfo中没有CaII谱线特征，这意味着Ca/Sr质量分数为<；0.002，这与电子分数≥0.40和每个核子的熵(以玻尔兹曼常数为单位)≥25的核合成是一致的。
Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound–bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr ii triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca ii triplet lines. This is natural because Ca and Sr are coproduced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb, and Th. Our results suggest that the line-forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr ii and Ca ii lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca ii line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is <0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction ≥0.40 and entropy per nucleon (in units of Boltzmann constant) ≥25.