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arXiv 1903.01262astro-ph.HE
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超新星遗迹shock waves氢谱线的辐射转移：2s态氢原子的贡献

Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms

Jiro Shimoda, J. Martin Laming

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16 pages, accepted for publication in MNRAS
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摘要：

考虑氢原子2s态布居的影响，研究了超新星遗迹（SNR）激波中氢谱线的辐射转移。巴耳末谱线发射的测量，特别是H~ $α$的测量，常常被用来推断SNR激波中的物理条件。另一方面，Lyman系列光子，特别是Ly~ $β$，大部分被上游的氢原子吸收。结果，原子被激发到3 p态，然后通过3 p到2s的自发跃迁发射出H~ $α$.因此，H~ $α$的性质取决于有多少Ly~ $β$光子被转换成H~ $α$光子。此外，巴耳末线可以被2s态氢原子散射，这些氢原子不仅被碰撞激发，而且被莱曼-巴耳末转换激发。例如，当激波传播到密度为$\sim30~{\rm cm^{-3}}$、大小为$\sim 1$ ~pc的H~ $_{\rm I}$云中时，H~ $α$光子被散射。我们发现H~ $α$的谱线轮廓由于3s到2 p、3 p到2s和3d到2 p跃迁的谱线中心频率不同而变得不对称。我们还发现，H~ $α$的宽窄比，这是经常用来估计离子-电子温度平衡，变化最多$\simeq 10$ %取决于上游介质的电离度，因为莱曼线到巴耳末线的不完全转换。

英文摘要：

Radiative transfer in hydrogen lines in supernova remnant (SNR) shock waves is studied taking into account the population of the hydrogen atom 2s-state. Measurements of Balmer line emission, especially of H~$α$, are often relied upon to derive physical conditions in the SNR shock. On the other hand, Lyman series photons, especially Ly~$β$, are mostly absorbed by upstream hydrogen atoms. As a result, atoms are excited to the 3p state, and then emit H~$α$ by the spontaneous transition from 3p to 2s. Thus, the nature of H~$α$ depends on how many Ly~$β$ photons are converted to H~$α$ photons. Moreover, the Balmer lines can be scattered by the 2s-state hydrogen atoms, which are excited not only by collisional excitation but also by the Lyman-Balmer conversion. It is shown for example that the H~$α$ photons are scattered if the shock propagates into an H~$_{\rm I}$ cloud with a density of $\sim30~{\rm cm^{-3}}$ and a size of $\sim 1$~pc. We find that the line profile of H~$α$ becomes asymmetric resulting from the difference between line centre frequencies among the transitions from 3s to 2p, from 3p to 2s and from 3d to 2p. We also find that the broad-to-narrow ratio of H~$α$, which is often used to estimate the ion-electron temperature equilibrium, varies at most $\simeq 10$ per cent depending on the ionization degree of the upstream medium because of incomplete conversion of Lyman lines to Balmer lines.

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