星际亚稳态氦吸收对Cosmic-Ray电离率的探测
Interstellar Metastable Helium Absorption as a Probe of the Cosmic-Ray Ionization Rate
摘要：
星际物质被宇宙射线电离的速率一直是争议的主要来源，不同的估计相差三个数量级。这个速率的观测限制都依赖于分析宇宙射线电离后产生的各种分子的化学性质，在许多情况下，这些分析包含显着的不确定性。即使在最简单的情况下（H3+），导出的电离率取决于（不确定的）估计的吸收路径长度。在本文中，我们研究的可能性推断宇宙射线电离率使用10830 A吸收线的亚稳态氦。通过扩散云对HD 183143的观测，但只产生亚稳氦柱密度的上限。对He+化学的彻底研究表明，只有一小部分He+会重新结合到三重态并填充亚稳能级。此外，必须考虑次级电子对氦的三重态流形的激发，因为它是在某些环境中产生He* 的主要机制。阐明这些不同的形成和破坏途径，我们推导出新的方程的亚稳态氦的稳态丰度。将这些方程与我们的观测结果相结合，我们发现zeta_He < 1.2*10^-15 s^-1，这个上限比之前用H3+在这条视线上推断的电离率大5倍。虽然目前对星际He* 的观测极其困难。
英文摘要：
The ionization rate of interstellar material by cosmic rays has been a major source of controversy, with different estimates varying by three orders of magnitude. Observational constraints of this rate have all depended on analyzing the chemistry of various molecules that are produced following cosmic-ray ionization, and in many cases these analyses contain significant uncertainties. Even in the simplest case (H3+) the derived ionization rate depends on an (uncertain) estimate of the absorption path length. In this paper we examine the feasibility of inferring the cosmic-ray ionization rate using the 10830 A absorption line of metastable helium. Observations through the diffuse clouds toward HD 183143 are presented, but yield only an upper limit on the metastable helium column density. A thorough investigation of He+ chemistry reveals that only a small fraction of He+ will recombine into the triplet state and populate the metastable level. In addition, excitation to the triplet manifold of helium by secondary electrons must be accounted for as it is the dominant mechanism which produces He* in some environments. Incorporating these various formation and destruction pathways, we derive new equations for the steady state abundance of metastable helium. Using these equations in concert with our observations, we find zeta_He < 1.2*10^-15 s^-1, an upper limit about 5 times larger than the ionization rate previously inferred for this sight line using H3+. While observations of interstellar He* are extremely difficult at present...
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