Lanthanide Features in Near-infrared Spectra of Kilonovae
Lanthanide Features in Near-infrared Spectra of Kilonovae
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DOI：
10.3847/1538-4357/ac8c36
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发表时间：
2022-06
期刊：
影响因子：
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通讯作者：
N. Domoto;Masaomi Tanaka;D. Kato;K. Kawaguchi;K. Hotokezaka;S. Wanajo
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文献类型：
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作者：
N. Domoto;Masaomi Tanaka;D. Kato;K. Kawaguchi;K. Hotokezaka;S. Wanajo
GW170817/AT2017gfo的观测为我们提供了双中子星合并是r过程核合成场所的证据。然而，在 GW170817/AT2017gfo 光谱中观察到的特征尚未完全解码，特别是在近红外 (NIR) 波长中。在本文中，我们研究了整个波长范围内的千新星光谱，目的是进行元素识别。我们通过构建一个混合线列表来系统地计算束缚-束缚跃迁的强度，该混合线列表对于重要的强跃迁是准确的，对于弱跃迁是完整的。我们发现元素周期表左侧的元素，如 Ca、Sr、Y、Zr、Ba、La 和 Ce，往往在光谱中产生明显的吸收线。这是因为此类元素的价电子数量较少且能级较低，从而导致强跃迁。通过对整个喷射物进行自洽辐射传输模拟，我们发现近红外光谱中出现了 La iii 和 Ce iii ，这可以解释 GW170817/AT2017gfo 光谱中 λ ∼ 12000–14000 Å 处的吸收特征。 La 和 Ce 的质量分数估计分别为 >2 × 10−6 和 ~(1–100) × 10−5。锕系元素 Th 也可以作为吸收源，因为其原子结构与 Ce 类似。然而，我们发现 Th iii 特征在光谱中不太突出，因为锕系元素的能级比镧系元素更密集。
The observations of GW170817/AT2017gfo have provided us with evidence that binary neutron star mergers are sites of r-process nucleosynthesis. However, the observed signatures in the spectra of GW170817/AT2017gfo have not been fully decoded, especially in the near-infrared (NIR) wavelengths. In this paper, we investigate the kilonova spectra over the entire wavelength range with the aim of elemental identification. We systematically calculate the strength of bound–bound transitions by constructing a hybrid line list that is accurate for important strong transitions and complete for weak transitions. We find that the elements on the left side of the periodic table, such as Ca, Sr, Y, Zr, Ba, La, and Ce, tend to produce prominent absorption lines in the spectra. This is because such elements have a small number of valence electrons and low-lying energy levels, resulting in strong transitions. By performing self-consistent radiative transfer simulations for the entire ejecta, we find that La iii and Ce iii appear in the NIR spectra, which can explain the absorption features at λ ∼ 12000–14000 Å in the spectra of GW170817/AT2017gfo. The mass fractions of La and Ce are estimated to be >2 × 10−6 and ∼(1–100) × 10−5, respectively. An actinide element Th can also be a source of absorption as the atomic structure is analogous to that of Ce. However, we show that Th iii features are less prominent in the spectra because of the denser energy levels of actinides compared to those of lanthanides.