氦氢离子（HeH $^+$）的首次天体物理探测
First astrophysical detection of the helium hydride ion (HeH$^+$)
摘要：
在化学的黎明时期，当年轻宇宙的温度降到$\sim$ 4000 K以下时，大爆炸核合成过程中产生的轻元素离子按照电离势的相反顺序重新组合。由于其较高的电离势，He $^{++}$（54.5 eV）和He $^+$（24.6 eV）首先与自由电子结合形成第一个中性原子，然后再与氢（13.6 eV）复合。当时，在这种无金属和低密度的环境中，中性氦原子通过与质子的辐射缔合形成了氦氢化物离子HeH $^+$中的宇宙第一个分子键（He + H $^+$ $\rightarrow$ HeH $^+$ + h $ν$）。随着重组的进行，HeH $^+$（HeH $^+$ + H $\rightarrow$ He + H $_2^+$）的破坏创造了形成分子氢的第一条途径，标志着分子时代的开始。尽管HeH $^+$分子对早期宇宙的演化具有毋庸置疑的重要性，但迄今为止，它在星际空间中没有被明确探测到。早在1925年，在实验室里就发现了这种离子，但直到70年代末，人们才开始讨论HeH $^+$存在于局部天体物理等离子体中的可能性。特别是，行星状星云的条件被证明适合于产生潜在可检测的HeH $^+$柱密度：来自中央热白矮星的硬辐射场产生重叠的斯特伦格伦球，HeH $^+$预计将在那里形成，主要通过He $^+$和H的辐射缔合。使用SOFIA上的GREAT光谱仪，现在可以获得$λ$ 149.1 $μ$ m处的HeH $^+$转动基态跃迁。我们在这里报告它对行星状星云NGC7027的探测。
英文摘要：
During the dawn of chemistry when the temperature of the young Universe had fallen below $\sim$4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He$^{++}$ (54.5 eV) and He$^+$ (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH$^+$, by radiative association with protons (He + H$^+$ $\rightarrow$ HeH$^+$ + h$ν$). As recombination progressed, the destruction of HeH$^+$ (HeH$^+$ + H $\rightarrow$ He + H$_2^+$) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH$^+$ molecule has so far escaped unequivocal detection in interstellar space. In the laboratory, the ion was discovered as long ago as 1925, but only in the late seventies was the possibility that HeH$^+$ might exist in local astrophysical plasmas discussed. In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH$^+$ column densities: the hard radiation field from the central hot white dwarf creates overlapping Strömgren spheres, where HeH$^+$ is predicted to form, primarily by radiative association of He$^+$ and H. With the GREAT spectrometer onboard SOFIA, the HeH$^+$ rotational ground-state transition at $λ$149.1 $μ$m is now accessible. We report here its detection towards the planetary nebula NGC7027.
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