Non-LTE analysis for Helium and Strontium lines in the kilonova AT2017gfo
Non-LTE analysis for Helium and Strontium lines in the kilonova AT2017gfo
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发表时间：
2023-02
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通讯作者：
Y. Tarumi;K. Hotokezaka;N. Domoto;Masaomi Tanaka
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作者：
Y. Tarumi;K. Hotokezaka;N. Domoto;Masaomi Tanaka
Kilonova光谱留下了有关中子星星合并中合成的元素的宝贵信息。在与GW 170817相关的千诺瓦（kilonova）AT 2017 gfo中，以8000埃为中心的光谱特征被解释为由单离子化锶产生的P-天鹅座轮廓。最近，佩雷戈等人（2022）提出氦10833线可以作为该特征的另一种解释。在这里，我们研究了非局部热力学平衡下的谱线特征。我们发现，通过阻止放射性衰变产物使喷出物电离，可以显著增强谱线形成区附近的电离态。我们计算的kilonova谱的假设下，球对称和均匀的元素分数的线形成的区域。我们发现，在1.43 ~ 4.40天的时间内，喷出物中0.2%（质量）的氦能再现观测光谱中的天鹅座P型特征。质量分数为1%的锶也能在1.5天左右产生吸收，但由于放射性衰变产物的电离作用，吸收随时间变弱。He线签名的强度敏感地取决于前两个时期的UV强度。进一步模拟紫外线的$r$-过程元素和光$r$-过程元素的光学特性的覆盖将是至关重要的氦和锶功能之间的区别。He的质量分数是一个很好的指标，喷出物熵，使我们能够探测质量喷射机制。
Kilonova spectra imprint valuable information about the elements synthesized in neutron star mergers. In AT2017gfo, the kilonova associated with GW170817, the spectroscopic feature centered around 8000 angstroms has been interpreted as the P-Cygni profile arising from singly ionized Strontium. Recently, Perego et al. (2022) suggested that Helium 10833 line can be an alternative explanation of the feature. Here, we study the line features under non-local thermodynamic equilibrium. We find that the ionization of ejecta by the stopping of radioactive decay product can significantly enhance the ionization states around the line forming region. We compute the kilonova spectrum under the assumption of spherical symmetry and uniform elemental fraction in the line-forming region. We find that 0.2\% (in mass) of Helium in the ejecta can reproduce the P-Cygni feature in the observed spectrum at $1.43$ -- $4.40$ days. Strontium with a mass fraction of $1\%$ is also able to make the absorption feature at $\sim 1.5\,$days, but it gets weaker with time due to ionization by radioactive decay products. The strength of the He line signature depends sensitively on UV strength for the first two epochs. Further modeling of UV line blanketing by $r$-process elements and the optical properties of light $r$-process elements would be crucial to distinguish between Helium and Strontium features. The mass fraction of He is a good indicator for ejecta entropy that allows us to probe the mass ejection mechanism.